Today I rebuilt a PC with new drives and all seemed OK, but after 15 minutes or so it kept stopping (no nice shutdown), irrespective of what the PC was doing. I even swapped back the old disks all to no avail, hence I was stumped for a while.
The problem turned out to be it was a somewhat full case and a wire was stopping the system fan turning, so the CPU was going into thermal shutdown and leaving no event log messages. Also the motherboard was not complaining as the am was still drawing current.
I should have spotted this one earlier, as it has happened before, so it goes on my blog of stupid things you forget to check for!