XP SHUTS DOWN BY ITSELF

My work computer running XP-pro
all of a sudden started shutting off by itself. I read many post where it was hardware related. I
ran: mem test, hdd test, mother board test, check for hardware conflicts / issues, all tested OK.
I had no way to check for the power supply and realize it could be the culprit. The next day I decided to start in safemode and run anti-virus and malware software, which found nothing. Now
after the reboot the **** thing
is running fine?? Any body else ever run across anything like this?