I think it may be linked to a PSU problem
I've had similar issue for my rig, and I've bought a 550W PSU + Add2PSU adaptor, and it's much more stable now
It's quite cheap, you should try : buy a PSU online, and if the problem is still here, send it back
That's my suspicion as well, but when I switch to mining Zcash, this problem does not come up and the power consumption when mining Zcash is about the same as when mining ethereum. So if it's the power supply, why is it only specific to ethereum mining?