Does it give you the blue screen? I had this before, it may not be the same thing but it may the hard disk or it generally overheating in the computer. Im no expert though as you know.......lol.
Mine would just reboot every twenty minutes or so after Id had it on a while. You could make sure the settings for the screensaver and power settings are not set to turn off the hard drives etc. The 'wake up' function used to reboot my machine rather than just pop back into windows. I set them all to 'never' on the power saving features.
See ya
Sirius.