Does this seem like a PSU issue?
I have a SeaSonic G Series SSR-550RM 550W that I've had to send in for a warranty repair (Seasonic was great about it) a year ago or so. After the repair, the system has been absolutely stable.
Until a few months ago.
In May, I added a GPU finally. It's a SAPPHIRE PULSE Radeon RX 5600 XT. The recommend PSU is 500w, minimum 450w.
Further, the random restarts mostly happen when the machine is idling at the desktop.
I did observe the restart once or twice. It isn't like pressing the reset button. The system shuts off, stays off for a few seconds, then turns back on. It will then stay on for another random amount of time. My latest uptime is 18h. I've gotten as long as a few days.
I don't have spare parts, and everything is out of warranty.