I have the same issue with my L3+. The miner starts showing some xxxx in aleatory hashboards.. I restart and it works fine again, sometimes the hashrate goes 0.00mhs.. restart and everything fine... now, it lost 1 of the hashboards.. I don't understand because the temperature it's fine..
The first two problems might occur because you do not have a stable amp. That could be because your outlet is shit, you run on 110V or simply because your PSU is either broken or some cables got loose.
I would try to unplug it and then re-plug everything. That might do the trick. You can also just lay the miner on the side that the cables don't bend
If that all does not help, you got a broken board and need to send it back to warranty (unless you OC it)