Random Power Loss on Dell XPS 600

System is an XPS 600 running XP Media Center Ed. that was build mainlyfor gaming. I bought the system new from Dell. A couple of days agoafter I booted the PC and went away for a few mins. When I came backit was powered dow. I rebooted it and it has run fine untill todaywhen the same thing happened. I rebooted and started up a game. Twominutes later the power went out (like I had pulled the power cord fromthe back of the machine, no warning). I have rebooted it again and ithas been fine for about an hour now, but I know it's a matter of time. What I have done:

Checked all connections that they are tight

Checked that the power strip is plugged in snug.

Cleaned any dust from the outside of the machine

The machine was opened and vacumed out about 6 months ago

Ido not thing it is heat related since it can happen just minutes afterboot up and then not for days. Suggestions or ideas? Any helpappreciated..