the setup is a bit messed:

router A, Buffalo WHR-G125, the main router, Tomato, set to static ip 192.168.1.1
router B, 941, WDS AP, set to static ip 192.168.1.2
router C, 941, WDS Client, set to static ip 192.168.1.3

A <-> B is connected by wire, from one LAN port on A to another LAN port on B
B <-> C is connected by WDS, slow but works fine

I deleted the WAN interface, disabled dnsmasq/firewall on B and C

the issue happens on eth0 of B, the wired connection to A:
Feb 27 18:14:12 clouds-ap-2 kern.info kernel: [  153.100000] eth0: link down
Feb 27 18:14:12 clouds-ap-2 kern.info kernel: [  153.100000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:14:12 clouds-ap-2 kern.info kernel: [  153.330000] eth0: link up (1000Mbps/Full duplex)
Feb 27 18:14:12 clouds-ap-2 kern.info kernel: [  153.330000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:14:12 clouds-ap-2 kern.info kernel: [  153.340000] br-lan: port 1(eth0) entering forwarding state

ping 192.168.1.2 (from my desktop wired to A) shows about 15% packet loss

I tried three different cables and the problem persists, and all those cables work fine in other situations
I tried to switch the role of B/C, the problem persists, so I guess its not a hardware issue
I tried to remove eth0 and add eth1 to br-lan, and use the WAN port on B instead , the problem... well, moved to eth1
I thought it might be a compatibility problem between A and B, I tried to setup a switch between them and the problem persists
I used backfire 10.3.1 at first and then updated to snapshot AA r30728, the problem persist

the system log is flooded by link down/up messages, I'll try to post the entire log after reboot

Feb 27 18:36:03 clouds-ap-2 syslog.info syslogd started: BusyBox v1.19.3
Feb 27 18:36:03 clouds-ap-2 kern.notice kernel: klogd started: BusyBox v1.19.3 (2012-02-25 20:09:07 MST)
Feb 27 18:36:03 clouds-ap-2 kern.notice kernel: [    0.000000] Linux version 3.2.5 (openwrt@OpenWRTBuild) (gcc version 4.6.3 20120201 (prerelease) (Linaro GCC 4.6-2012.02) ) #1 Sun Feb 26 08:04:05 MST 2012
Feb 27 18:36:03 clouds-ap-2 kern.debug kernel: [    0.000000] MyLoader: sysp=ffffffff, boardp=ffffffff, parts=f0f0f0f0
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.000000] bootconsole [early0] enabled
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.000000] CPU revision is: 00019374 (MIPS 24Kc)
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.000000] SoC: Atheros AR7240 rev 2
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.000000] Clocks: CPU:400.000MHz, DDR:400.000MHz, AHB:200.000MHz, Ref:5.000MHz
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.000000] Determined physical RAM map:
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.000000]  memory: 02000000 @ 00000000 (usable)
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.000000] Initrd not found or empty - disabling initrd
Feb 27 18:36:03 clouds-ap-2 kern.warn kernel: [    0.000000] Zone PFN ranges:
Feb 27 18:36:03 clouds-ap-2 kern.warn kernel: [    0.000000]   Normal   0x00000000 -> 0x00002000
Feb 27 18:36:03 clouds-ap-2 kern.warn kernel: [    0.000000] Movable zone start PFN for each node
Feb 27 18:36:03 clouds-ap-2 kern.warn kernel: [    0.000000] early_node_map[1] active PFN ranges
Feb 27 18:36:03 clouds-ap-2 kern.warn kernel: [    0.000000]     0: 0x00000000 -> 0x00002000
Feb 27 18:36:03 clouds-ap-2 kern.debug kernel: [    0.000000] On node 0 totalpages: 8192
Feb 27 18:36:03 clouds-ap-2 kern.debug kernel: [    0.000000] free_area_init_node: node 0, pgdat 802e0cf0, node_mem_map 81000000
Feb 27 18:36:03 clouds-ap-2 kern.debug kernel: [    0.000000]   Normal zone: 64 pages used for memmap
Feb 27 18:36:03 clouds-ap-2 kern.debug kernel: [    0.000000]   Normal zone: 0 pages reserved
Feb 27 18:36:03 clouds-ap-2 kern.debug kernel: [    0.000000]   Normal zone: 8128 pages, LIFO batch:0
Feb 27 18:36:03 clouds-ap-2 kern.debug kernel: [    0.000000] pcpu-alloc: s0 r0 d32768 u32768 alloc=1*32768
Feb 27 18:36:03 clouds-ap-2 kern.debug kernel: [    0.000000] pcpu-alloc: [0] 0
Feb 27 18:36:03 clouds-ap-2 kern.warn kernel: [    0.000000] Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 8128
Feb 27 18:36:03 clouds-ap-2 kern.notice kernel: [    0.000000] Kernel command line:  board=TL-WR741ND console=ttyS0,115200 rootfstype=squashfs,jffs2 noinitrd
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.000000] PID hash table entries: 128 (order: -3, 512 bytes)
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.000000] Dentry cache hash table entries: 4096 (order: 2, 16384 bytes)
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.000000] Inode-cache hash table entries: 2048 (order: 1, 8192 bytes)
Feb 27 18:36:03 clouds-ap-2 kern.warn kernel: [    0.000000] Primary instruction cache 64kB, VIPT, 4-way, linesize 32 bytes.
Feb 27 18:36:03 clouds-ap-2 kern.warn kernel: [    0.000000] Primary data cache 32kB, 4-way, VIPT, cache aliases, linesize 32 bytes
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.000000] Writing ErrCtl register=00000000
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.000000] Readback ErrCtl register=00000000
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.000000] Memory: 29196k/32768k available (2165k kernel code, 3572k reserved, 399k data, 196k init, 0k highmem)
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.000000] SLUB: Genslabs=9, HWalign=32, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.000000] NR_IRQS:48
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.000000] Calibrating delay loop... 265.42 BogoMIPS (lpj=1327104)
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.080000] pid_max: default: 32768 minimum: 301
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.080000] Mount-cache hash table entries: 512
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.090000] NET: Registered protocol family 16
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.090000] MIPS: machine is TP-LINK TL-WR741ND
Feb 27 18:36:03 clouds-ap-2 kern.warn kernel: [    0.500000] registering PCI controller with io_map_base unset
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.510000] bio: create slab <bio-0> at 0
Feb 27 18:36:03 clouds-ap-2 kern.debug kernel: [    0.520000] pci 0000:00:00.0: [168c:002a] type 0 class 0x000280
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.520000] pci 0000:00:00.0: fixup device configuration
Feb 27 18:36:03 clouds-ap-2 kern.debug kernel: [    0.520000] pci 0000:00:00.0: reg 10: [mem 0x00000000-0x0000ffff 64bit]
Feb 27 18:36:03 clouds-ap-2 kern.debug kernel: [    0.520000] pci 0000:00:00.0: supports D1
Feb 27 18:36:03 clouds-ap-2 kern.debug kernel: [    0.520000] pci 0000:00:00.0: PME# supported from D0 D1 D3hot
Feb 27 18:36:03 clouds-ap-2 kern.debug kernel: [    0.520000] pci 0000:00:00.0: PME# disabled
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.520000] pci 0000:00:00.0: BAR 0: assigned [mem 0x10000000-0x1000ffff 64bit]
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.530000] pci 0000:00:00.0: BAR 0: set to [mem 0x10000000-0x1000ffff 64bit] (PCI address [0x10000000-0x1000ffff])
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.540000] pci 0000:00:00.0: using irq 40 for pin 1
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.540000] Switching to clocksource MIPS
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.550000] NET: Registered protocol family 2
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.550000] IP route cache hash table entries: 1024 (order: 0, 4096 bytes)
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.550000] TCP established hash table entries: 1024 (order: 1, 8192 bytes)
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.560000] TCP bind hash table entries: 1024 (order: 0, 4096 bytes)
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.560000] TCP: Hash tables configured (established 1024 bind 1024)
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.570000] TCP reno registered
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.570000] UDP hash table entries: 256 (order: 0, 4096 bytes)
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.580000] UDP-Lite hash table entries: 256 (order: 0, 4096 bytes)
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.590000] NET: Registered protocol family 1
Feb 27 18:36:03 clouds-ap-2 kern.debug kernel: [    0.590000] PCI: CLS 0 bytes, default 32
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.610000] squashfs: version 4.0 (2009/01/31) Phillip Lougher
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.620000] JFFS2 version 2.2 (NAND) (SUMMARY) (LZMA) (RTIME) (CMODE_PRIORITY) (c) 2001-2006 Red Hat, Inc.
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.630000] msgmni has been set to 57
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.630000] io scheduler noop registered
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.640000] io scheduler deadline registered (default)
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.640000] Serial: 8250/16550 driver, 1 ports, IRQ sharing disabled
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.670000] serial8250.0: ttyS0 at MMIO 0x18020000 (irq = 11) is a 16550A
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.680000] console [ttyS0] enabled, bootconsole disabled
Feb 27 18:36:03 clouds-ap-2 kern.warn kernel: [    0.690000] m25p80 spi0.0: found s25sl032a, expected m25p80
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.700000] m25p80 spi0.0: s25sl032a (4096 Kbytes)
Feb 27 18:36:03 clouds-ap-2 kern.notice kernel: [    0.700000] 5 tp-link partitions found on MTD device spi0.0
Feb 27 18:36:03 clouds-ap-2 kern.notice kernel: [    0.710000] Creating 5 MTD partitions on "spi0.0":
Feb 27 18:36:03 clouds-ap-2 kern.notice kernel: [    0.710000] 0x000000000000-0x000000020000 : "u-boot"
Feb 27 18:36:03 clouds-ap-2 kern.notice kernel: [    0.720000] 0x000000020000-0x000000105e00 : "kernel"
Feb 27 18:36:03 clouds-ap-2 kern.warn kernel: [    0.730000] mtd: partition "kernel" must either start or end on erase block boundary or be smaller than an erase block -- forcing read-only
Feb 27 18:36:03 clouds-ap-2 kern.notice kernel: [    0.740000] 0x000000105e00-0x0000003f0000 : "rootfs"
Feb 27 18:36:03 clouds-ap-2 kern.warn kernel: [    0.740000] mtd: partition "rootfs" must either start or end on erase block boundary or be smaller than an erase block -- forcing read-only
Feb 27 18:36:03 clouds-ap-2 kern.notice kernel: [    0.760000] mtd: partition "rootfs" set to be root filesystem
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    0.770000] mtd: partition "rootfs_data" created automatically, ofs=270000, len=180000
Feb 27 18:36:03 clouds-ap-2 kern.notice kernel: [    0.770000] 0x000000270000-0x0000003f0000 : "rootfs_data"
Feb 27 18:36:03 clouds-ap-2 kern.notice kernel: [    0.780000] 0x0000003f0000-0x000000400000 : "art"
Feb 27 18:36:03 clouds-ap-2 kern.notice kernel: [    0.790000] 0x000000020000-0x0000003f0000 : "firmware"
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    1.140000] ag71xx_mdio: probed
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    1.140000] eth0: Atheros AG71xx at 0xba000000, irq 5
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    1.730000] eth0: Found an AR7240/AR9330 built-in switch
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    1.750000] eth1: Atheros AG71xx at 0xb9000000, irq 4
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    2.360000] ag71xx ag71xx.0: eth1: connected to PHY at ag71xx-mdio.0:04 [uid=004dd041, driver=Generic PHY]
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    2.370000] TCP westwood registered
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    2.370000] NET: Registered protocol family 17
Feb 27 18:36:03 clouds-ap-2 kern.notice kernel: [    2.370000] Bridge firewalling registered
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    2.380000] 8021q: 802.1Q VLAN Support v1.8
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    2.400000] VFS: Mounted root (squashfs filesystem) readonly on device 31:2.
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    2.410000] Freeing unused kernel memory: 196k freed
Feb 27 18:36:03 clouds-ap-2 kern.info kernel: [    6.220000] eth0: link up (1000Mbps/Full duplex)
Feb 27 18:36:03 clouds-ap-2 kern.debug kernel: [    6.370000] Registered led device: tp-link:green:lan1
Feb 27 18:36:04 clouds-ap-2 user.notice ifup: Enabling Router Solicitations on loopback (lo)
Feb 27 18:36:04 clouds-ap-2 kern.info kernel: [   16.450000] device eth0 entered promiscuous mode
Feb 27 18:36:04 clouds-ap-2 kern.info kernel: [   16.490000] eth0: link up (1000Mbps/Full duplex)
Feb 27 18:36:04 clouds-ap-2 kern.info kernel: [   16.510000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:04 clouds-ap-2 kern.info kernel: [   16.520000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:06 clouds-ap-2 kern.info kernel: [   18.660000] device wlan0 entered promiscuous mode
Feb 27 18:36:06 clouds-ap-2 kern.info kernel: [   18.690000] br-lan: port 2(wlan0) entering forwarding state
Feb 27 18:36:06 clouds-ap-2 kern.info kernel: [   18.700000] br-lan: port 2(wlan0) entering forwarding state
Feb 27 18:36:06 clouds-ap-2 kern.info kernel: [   18.780000] device eth0 left promiscuous mode
Feb 27 18:36:06 clouds-ap-2 kern.info kernel: [   18.790000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:07 clouds-ap-2 user.notice ifup: Enabling Router Solicitations on loopback (lo)
Feb 27 18:36:07 clouds-ap-2 kern.info kernel: [   19.200000] device eth0 entered promiscuous mode
Feb 27 18:36:07 clouds-ap-2 kern.info kernel: [   19.200000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:07 clouds-ap-2 kern.info kernel: [   19.210000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:09 clouds-ap-2 user.notice ifup: Enabling Router Solicitations on lan (br-lan)
Feb 27 18:36:09 clouds-ap-2 kern.info kernel: [   21.710000] device wlan0 left promiscuous mode
Feb 27 18:36:09 clouds-ap-2 kern.info kernel: [   21.720000] br-lan: port 2(wlan0) entering forwarding state
Feb 27 18:36:10 clouds-ap-2 kern.info kernel: [   22.240000] device wlan0 entered promiscuous mode
Feb 27 18:36:10 clouds-ap-2 kern.info kernel: [   22.240000] br-lan: port 2(wlan0) entering forwarding state
Feb 27 18:36:10 clouds-ap-2 kern.info kernel: [   22.250000] br-lan: port 2(wlan0) entering forwarding state
Feb 27 18:36:11 clouds-ap-2 authpriv.info dropbear[1117]: Running in background
Feb 27 18:36:12 clouds-ap-2 daemon.info hostapd: wlan0: STA 60:fa:cd:58:04:39 IEEE 802.11: authenticated
Feb 27 18:36:12 clouds-ap-2 daemon.info hostapd: wlan0: STA 60:fa:cd:58:04:39 IEEE 802.11: associated (aid 1)
Feb 27 18:36:12 clouds-ap-2 user.info sysinit: setting up led WAN
Feb 27 18:36:12 clouds-ap-2 user.info sysinit: setting up led LAN1
Feb 27 18:36:12 clouds-ap-2 user.info sysinit: setting up led LAN2
Feb 27 18:36:12 clouds-ap-2 user.info sysinit: setting up led LAN3
Feb 27 18:36:12 clouds-ap-2 user.info sysinit: setting up led LAN4
Feb 27 18:36:12 clouds-ap-2 kern.info kernel: [   24.500000] eth0: link down
Feb 27 18:36:12 clouds-ap-2 kern.info kernel: [   24.500000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:12 clouds-ap-2 kern.info kernel: [   24.700000] eth0: link up (1000Mbps/Full duplex)
Feb 27 18:36:12 clouds-ap-2 kern.info kernel: [   24.700000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:12 clouds-ap-2 kern.info kernel: [   24.710000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:15 clouds-ap-2 daemon.info hostapd: wlan0: STA 00:1c:26:bc:b9:b7 IEEE 802.11: authenticated
Feb 27 18:36:15 clouds-ap-2 daemon.info hostapd: wlan0: STA 00:1c:26:bc:b9:b7 IEEE 802.11: associated (aid 1)
Feb 27 18:36:15 clouds-ap-2 daemon.info hostapd: wlan0: STA 00:1c:26:bc:b9:b7 WPA: pairwise key handshake completed (RSN)
Feb 27 18:36:16 clouds-ap-2 kern.info kernel: [   28.540000] eth0: link down
Feb 27 18:36:16 clouds-ap-2 kern.info kernel: [   28.550000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:16 clouds-ap-2 kern.info kernel: [   28.760000] eth0: link up (1000Mbps/Full duplex)
Feb 27 18:36:16 clouds-ap-2 kern.info kernel: [   28.760000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:16 clouds-ap-2 kern.info kernel: [   28.770000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:22 clouds-ap-2 kern.info kernel: [   34.510000] eth0: link down
Feb 27 18:36:22 clouds-ap-2 kern.info kernel: [   34.510000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:22 clouds-ap-2 kern.info kernel: [   34.560000] eth0: link up (1000Mbps/Full duplex)
Feb 27 18:36:22 clouds-ap-2 kern.info kernel: [   34.560000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:22 clouds-ap-2 kern.info kernel: [   34.570000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:23 clouds-ap-2 kern.info kernel: [   35.590000] eth0: link down
Feb 27 18:36:23 clouds-ap-2 kern.info kernel: [   35.600000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:23 clouds-ap-2 kern.info kernel: [   35.720000] eth0: link up (1000Mbps/Full duplex)
Feb 27 18:36:23 clouds-ap-2 kern.info kernel: [   35.720000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:23 clouds-ap-2 kern.info kernel: [   35.730000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:37 clouds-ap-2 kern.info kernel: [   49.740000] eth0: link down
Feb 27 18:36:37 clouds-ap-2 kern.info kernel: [   49.740000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:38 clouds-ap-2 kern.info kernel: [   50.180000] eth0: link up (1000Mbps/Full duplex)
Feb 27 18:36:38 clouds-ap-2 kern.info kernel: [   50.180000] br-lan: port 1(eth0) entering forwarding state
Feb 27 18:36:38 clouds-ap-2 kern.info kernel: [   50.190000] br-lan: port 1(eth0) entering forwarding state

I tried to bridge A and B by connecting them to a desktop with two ethernet adapters, and use windows to bridge, the problem persists.

if I do not setup the bridge in windows, just connect the cable, there is no more link down message