Might not be the power supply, though that would have to be tested. Set up monitoring on your voltages, could be dipping below 11.4 volts and tripping undervoltage.
Check your PCIe power cables, if you can run independent 8-pin connectors rather than ones that split into multiple, that is always ideal. Even if you have to leave a whole bunch of PCIe connectors dangling (until you can pick up some straight through cables)
Have you checked your outlet and line? Make sure nothing of significance is also on the circuit for one thing. Even check the temperatures of your cords. If they are getting warm, you could be seeing enough voltage drop to cause an undervolt condition on PSU input. Last resort would be a line conditioner/UPS if you aren't able to verify potential problems. That would prevent sags in the line or spikes from the GPU from bringing you down. A problem at my parents house many years ago was the AC kicking on, used to trip the UPS we had on the family PC (And made the CRT wobble while it was running)