Core temp stays around 59C, the case has excellent air flow, 2x 120mm intake, 2x 120mm exhaust. I don't believe it's from overheating. That particular GPU was in a different system with much less air flow with higher temp around 72-78C and never happened. I'll swap another GPU in to see if it happens again.

On one particular system I have, the screen will go black at very random times. The system has one GPU mining ETH, I think it's mining every time it happens but I can't remember for sure.

System:
Pentium G4560 (new, 2 weeks old)
8GB Corsair Vengeance LPX 2400 CL14 (1 module, new 2 weeks old)
MSI Z270M MORTAR (refurbished, bought 2 weeks ago) <- ?? should I refund ??
EVGA 520B (1 year old, bought new)
XFX RX 470 4GB (Elpida) stock & modded black screen occurs, possibly undervolted every time, can't remember. (-96mV MSI Afterburner)
Kingston 120GB V300 SSD

What do you think the problem is, and how can I further isolate the problem? It happens about once a day or every other day. Happened earlier when I got up from the PC to leave the room.

*I've reinstalled Windows at least twice, reinstalled drivers countless times dealing with the auto-update in Windows 10. Happens on newest Crimson drivers and legacy drivers.