While it sounds like a overheating problem, PSU failure can sometimes produce those symptoms. You may be able to discern if your problem is overheating or PSU by measuring time interval between restart and black screen after allowing to sit with no power for 15 minutes. If the interval is fairly constant it is probably overheating - if not I'd suspect the PSU. If you have a spare PSU available you might try swapping PSUs - easy to do 3 or 4 connections (2 power connectors on mobo and 1 to each HDD/DVD drive.
You haven't given much in the way of details about your hardware - it would help those who are trying to help you if you did.
You might also try restoring to last restore point where this didn't happen.