HEAT - Take off the cover and blow a fan into the case, see if the problem goes away.
Look at your HSF and see if it is clogged with lint/dirt.
Are all the fans working in the case?
Is the fan on the Video card turning if it has one?
Check the PSU fan and see if it is turning.
Look at the capacitors on the mainboard. Any of the them leaking/bulging?
Tried installing new video drivers?
Looked for any spyware?
What antivirus are you running?
Go to MyComputer ICON right click it and go to Properties -> go to Advanced TAB -> under Startup and Recovery click on SETTINGS -> where it says System Failure uncheck Automatically restart.
This will give you a bsod if it is a software crash, might get some info and clues as to what is causing it.