Your system may be overheating. If this is the case, the motherboard is turning the system off in order to prevent your system from frying itself.
- Are all your fans running?
- Is the fan on your processor spinning at full speed?
- Does your case have proper ventilation (fans that intake air and exhaust it out the back)?
- Have you removed the heatsink from the CPU and reinstalled it lately? If so, did you reinstall it correct (cleaned off the old thermal compound, put a very thin layer of new thermal compound on the chip, and very carefully replaced the heatsink facing the correct direction)?
- If you open up the side of the case and point a very powerful fan into it and then start up the machine, does it run a bit longer before restarting?