W8970 Ethernet ports 3-4 stops working after a while

I've noticed that ethernet ports 1-2 always work and 3-4 stops working after hours.
The leds won't power on even if connecting multiple short ethernet cables. The only thing I can do is to reboot. After that they start working again until ~10h or less.

it happens irrespectively to the vlan functionality.

port 1: gigabit lan ( 2m cable ) untagged
port 2: gigabit lan ( ~10m cable ) untagged
port 3: 100mbps ( ~25m cable ) untagged
port 4: empty untagged

if I switch port 2 and 3 it's always port 3 that has the problem. so it's not the 100mbps long cable.

Model TDW8970 - TP-LINK TD-W8970
Firmware Version LEDE Reboot CURRENT r2446-9791fb2ac2 / LuCI Master (git-16.345.45270-e65f6512e)
Kernel Version 4.4.36

It may be a lede bug. kernel or whatever.
I'm searching for help here before posting a bug in the bugtracker