Don't know if this is any use but I had a similar problem.
I took my pc to the shop and they said that it was ok, couldn't find any hardware problems. I took it back reinstalled and blue screen, auto shut downs every 10-20 min depending on what was running.
It turned out to be a heat problem, the case i had was too small and not enough air circulating, I bought a new case and some new fans and that resolved
the Cd that came with my motherboard had a utility that measured system temp etc so your's might too if not this might help
http://www.almico.com/speedfan.php
it's a shot in the dark but it might help