Hi there,
Just out of curiosity are you using CoreTemp for temp monitoring, I’ve found that I was having a similar problem, and could find no explanation.
Like you the PC would just completely swithc off for no reason, almost as if the power cord had been pulled from the PSU.
Like you I thought it may be PSU related or more precisely my house power supply and possible power surges as I live within 1/2 mile from 2 industrial plants, and have often noticed my lights flickering…well not so much of a flicker as almost a half-second dimming…so I bought a really good surge protector, and it didn’t happen for a while my PC that is the lights still dim…lol.
But then all of a sudden it started doing it again, it took me a while to realize it… but eventually I noticed that it was only ever happening while I was running CoreTemp over a long period of time.
So now whenever I use CoreTemp, I open it get my temp readings, then immediately close it down, and doing it like this I had no more problems…
So just to test the theory I tried opening CoreTemp and leaving it running, and sure enough it started again…
So I’ve swapped over to using Everest Ultimate Edition now and have had no repeats of the problem since.
I’ve got no idea why CoreTemp could be causing this, but it was definitely the cause of my shutdowns.
PS: When running CoreTemp continuously sometimes it would run for hours without a problem, but then seemingly totally randomly it would power off the PC…!!!
I’m not saying its definitely the reason for yours, but before you go spending £££ on a new PSU, you may want to check this first.
BTW: Everest is much better I can set it up to display all system specs CPU use, Mem Useage, core temps, voltages, speeds etc on the LCD display on my speakers, so at a glance I can see all my temps even if the screen is switched off…cool..!!!..but it is quite memory intensive using about 15Mb of RAM its the 4th biggest memory process on my PC, with only my Internet Browser, MSN Messenger and Desktop Window Manager using more than it…but hey I got plenty to psare with 2Gb… 
Good luck,… let us know if this was the case perhaps its a common problem, I just thought it was a weird random thing to do with my setup, and it may well be…so it would be interesting to know if you also have CoreTemp and are having the same problem…!!!
Thanks
Kaz
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