New
#11
Memtest86 is good, but no RAM tester can detect all possible RAM failures. I would suggest that you adopt a slow and careful step-by-step approach to troubleshooting this clear hardware issue.
I would suggest that you strip the PC to the bare minimum. Unplug all external devices except monitor, mouse, and keyboard. You've already removed the GPU, so remove all but one RAM stick (and try other RAM sticks too). Remove unnecessary PCIe cards (extra USB ports cards, WiFi cards, for example). If it fails in as stripped-down version as you can achieve then I'd look at the PSU, and then the motherboard.
This isn't a software issue though.