Weird necros Larry. Is this a cry for help?
Not really, just wanted Dave to know that he was right, the system was overheating.
CPU fan was clogged with dust. A cloud of dust came out of the PSU too.
I had mistakenly thought that the PSU was failing after 4-5 years, but it looks (after cleaning it up, and stress-testing it), that that's not the case.
The BIOS was set to shutdown at 75C, so I guess with the accumulated dust clogging the CPU heatsink, it hit that mark occasionally during heavy use.