For about a year now I've had an issue when running any sort of heavy load on my PC (gaming, stress tests) that causes it to just power off and back on with no warning or BSOD. I have replaced the power supply and the problem still occurs. I have had one major upgrade in the past year, which was my graphics card (from a GTX 1060 to an RTX 3080 FTW3 Ultra Gaming. I hoped the problem might have just been an old/underpowered PSU, hence the power supply replacement, but no luck. I've had the rest of the parts for about 2 years.
I have been troubleshooting for the past few days, running