Well, my PSU finally arrived. *phew*
It's not exactly exactly what I had before, I guess it's a newer version of it. I gotta say, case cable management systems/arrangements can make it a PITA to switch a PSU out. >.>
Did the zombie game (the most reliable reboot offender by far), tried to stress-play it. A few hours, no reboot. Tried loading saves on a couple other games too. Not 100% sure yet, but hopefully that was the problem. The new PSU has this knob fan-speed control on the back by the power button, so you can make it silent or super-sonic sounding. Which got me thinking - maybe the fan on the old one stopped working -it was always super quiet but had no fan speed knob, so I assume it was auto or had a low factory default rpm - and because it was always essentially silent, I never noticed (can't see PSU fan when it's in the case, at all). eg, maybe over heating and damaging itself for a while etc. There was no electronic burning smell I ever noticed, but whatever.
... hopefully I can now put off a new PC for a year or two, like I wanted to. So maybe yay?
...still odd my RAM showed as having errors tho, so glad I replaced that too, in the end.