PC reboots before OS login

[Ianesz]

This issue occurred from one day to the next, with no intermediate issues of any kind. Basically the PC will simply reboot in either before (in most cases) the OS login screen is reached, or immediately after a login is attempted.
I've tried both on my primary, Linux install, as well as on my Windows install. The only difference seems to be that with Windows, I can consistently reach login screen, where that's rarely the case with Linux.

Components:
- i5 13600K (with Noctua NH-D15S)

- Gigabyte Z790 Aero G

- 2x48GB GSkill DDR5 6400

- PNY 1080Ti

- 1TB Samsung 980PRO + 2TB Samsung 980PRO

- Corsair HX850 from 2013

Here's what troubleshooting I did so far:
- Reset the BIOS to optimized defaults and then lowered the Gigabyte preset to the lowest available (basically the one that doesn't yet disable the E-Cores)

- Disabled XMP

- Removed one stick of RAM

- Swapped the remaining stick with the other

- Removed the dGPU, and using only iGPU

None of it made any difference.

It seems to me the most likely remaining culprits are PSU, CPU and MB (in that order).

The PSU, while best in class at the time of purchase, is definitely very old at this point in time. That's why I figured I'd just get an HX1200i as the next step, since I've been thinking of maybe upgrading the GPU at some point anyway, and I would be comfortable using an 850W that old anymore.

Normally, I'd never consider the CPU being a possible point of failure, but with Intel  13/14 gen degradation fiasco, I'm not leaving it out. That said it would be very odd, since I've had zero instability issues so far, and the 600 series was reportedly unaffected last I heard.

Anyone have an opinion on the matter?