my R7800 reboots randomly every couple of days.
Now it is running OpenWrt 19.07.7 r11306-c4a6851c72
But the reboots occured also with older versions.
As I used OpenWRT right from the start, I have no experience with the reliability of the stock firmware.
No fancy things configured:
4 VLANs, DHCP server for one of them.
I tried to use non-ct WiFi Drivers but the problem was not solved.
I plugged in an USB-stick as syslog-target and logged the last reboot:
Thu Mar 18 12:26:24 2021 daemon.notice netifd: wan (1464): udhcpc: lease of 31.x.y.z obtained, lease time 5400
Thu Mar 18 12:28:34 2021 kern.alert kernel: [911881.851730] Unable to handle kernel NULL pointer dereference at virtual address 00000000
Thu Mar 18 12:28:34 2021 kern.alert kernel: [911881.851774] pgd = c0204000
Thu Mar 18 12:28:34 2021 kern.alert kernel: [911881.859063]  *pgd=00000000
Thu Mar 18 12:28:34 2021 kern.emerg kernel: [911881.861642] Internal error: Oops: 80000007 [#1] SMP ARM
My two R7800 had similar behavior. After a few days (1 week max.) they rebooted for no apparent reason.
Just by stupid coincidence my problems went away after experimenting with the performance governor.
After that both R7800 are rock solid (currently 30 days running 19.07.7).
Hope this also helps you.
My rc.local (luci-system-startup-locaal startup) looks like this:
# Put your custom commands here that should be executed once
# the system init finished. By default this file does nothing.
# Ondemand Governor Tweaks
echo ondemand > /sys/devices/system/cpu/cpufreq/policy0/scaling_governor
echo ondemand > /sys/devices/system/cpu/cpufreq/policy1/scaling_governor
echo 800000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq
echo 800000 > /sys/devices/system/cpu/cpu1/cpufreq/scaling_min_freq
echo 35 > /sys/devices/system/cpu/cpufreq/ondemand/up_threshold
echo 10 > /sys/devices/system/cpu/cpufreq/ondemand/sampling_down_factor
# echo 75 > /sys/devices/system/cpu/cpufreq/ondemand/up_threshold
# echo 10 > /sys/devices/system/cpu/cpufreq/ondemand/sampling_down_factor
echo 1000000 > /sys/devices/system/cpu/cpufreq/ondemand/sampling_rate
# Performance Governor
# echo performance > /sys/devices/system/cpu/cpufreq/policy0/scaling_governor
# echo performance > /sys/devices/system/cpu/cpufreq/policy1/scaling_governor