(Redirected) Precision 370 Desktop w/ SCSI internal HD, WinXP

Service Tag [ Admin note, service tag removed] TD.   Computer startup progresses successfully through the BIOS screen, the Dell 'splash' screen and displays the loaded desktop showing shortcuts to my programs and files. I am able to move icons and open programs and folders - full control at this point. At approx 40 secs after the desktop screen loads, the monitor goes black. The mini tower still show a green indicator light of the front of the computer, and both power supply and the cooling fans are running - moving air. If I immediately try to re-start the computer, nothing happens - no screen display at all. However, If I wait several hours, I am able to duplicate the start-up conditions mentioned above. My first thoughts were that of a overheating problem...

No hardware or software changes / purchased from Dell 4/21/2005

Running Windows XP

I have replaced the power supply module, the cooling fan (blows air over the chipset and heatsink) and the the video card.

Any ideas and/suggestions?