I'd be surprised if your PSU was the problem because if a card draws more than a PSU can provide, it results in a power-off system reset.
I say this from experience. When I bought my first RX 5700 XT, it was an XFX Triple-Dissipation model. I was getting power-off resets during gaming and had no idea why because I knew that driver crashes don't cause the PC to reset. When I put my old Sapphire R9 Fury Nitro+ back in, all instability disappeared (I was still using the same driver installation). I sent the card back to XFX and they replaced it with a THICC-III model. I immediately noticed when I was installing it that the auxilliary PCIe power connectors were different. Where the Triple-Dissipation card had an 8+6-pin connector design, the THICC-III had 8+8. It dawned on me that my first 5700 XT was probably trying to pull more power than the 8+6-pin configuration could provide but not more than 8+8 could. My PSU was the same EVGA 1000 G2 Supernova 1kW PSU that I currently use today with my RX 7900 XTX so I knew that there was plenty of power on tap.
I honestly think that your card is probably worn-out. I remember that RDNA1 was very effective at mining Ethereum. Mining doesn't really wear GPUs out as long as they were properly taken care of. Unfortunately, a lot of the time, this was not the case. So, you might have purchased an ex-mining card that wasn't taken properly taken care of. I hope that it's not the case but your results tell me that the card is probably dying.