I have a last model G5 with dual 2.5ghz and 4.5 gigs of ram. It also has the 7800gt video card. After I start it up it runs for about 15 minutes and then the display goes blank and the system fans go full out. I've tried resetting the PRAM, PMU, VRAM, all of those things...the last thing I have is the video card to test. Also the power supply was just replaced by a Apple Repair Center under extended warranty. Any ideas guys?