Question Need help finding the component killing my graphics cards

[DrKobo]

1. MOBO Model: ASRock X470 Master SLI/AC
2. CPU: AMD Ryzen 2700 8-core 3.2GHz
3. Memory: G. Skill TridentZ RGB Series 16GB DDR4 3200
4. Video Card: XFX Radeon RX 580 XXX Edition 8GB
5. Operating System: Windows 10 Home SP 1909
6. SDD: Samsung 860 EVO 500 GB
7. HDD: Seagate BarraCuda ST2000DM006 2TB 7200rpm
8. PSU: EVGA G3 750W

I've had two cards die in this system: the original RX580 and an old known good HD7950 that died after a day of use. The original RX580 seemed to die out of the blue during a gaming session, no BSOD or crash just straight up complete power loss. The internals were inspected and cables and card reseated, but no apparent cause of failure as the system failed to even power on. The HD7950 seemed to work and I was able to use the system normally for a day, but then it suffered from intermittent display blackouts and graphical artifacts. Again, checked components and cables. I even put the two graphics cards in another system, which produced the exact same results for each card. At this point my suspicions turned to either the MOBO or PSU. Any suggestions on what to do now? Should I just try to RMA those two components?

 

[Mezoxin]

if i where you i would RMA these 2 components , how was the thermals in your case , and how is the PSU ventilation in particular ?