But that is one of the usual shop hounds. Sometimes you can fix it with a good cleaning and fresh heatsink along with updates to BIOS and more but after you do all that you start putting in a new PSU (make it bigger and single rail) then motherboard, CPU and RAM.

The build is sadly one I've seen for years and cringe as I think about how often it takes a lot to fix it.

So, start with the cheap work and DO NOT FALL for temp measurements. Just do the heatsink and compound works, BIOS and be sure the BIOS is in defaults or it's safest settings.