Random Video Crashes

Okay, I'm a computer Technician, and this is a problem I encountered on my personal box, that I have never seen before..., Okay, here's how it goes, when I turn my computer on, it runs fine, for about 10-15 minutes, then there seems to be a video lock up, and my screen goes blank, but, the hardrives, power supply, everything else is still going. I have tried three different video cards, and countless different combinations of ram, including trying all my chips one at a time. I even installed my OS again, and was faced with the same problem, I've also tried different harddrives. Here are the specs, Asus K8v Motherboard, with socket 754, Athlon 2800+ running at the reccomended 1.8 GHZ, ATI Radeon 9800 Video Card, 1.5 gigs ***orted ram (Ultra, Kingston, Crucial). 2 120 Gig Maxtor Harddrives, and 1 160 gig maxtor, HP DVD+Burner, and a LiteOn 56x Cd Writer. I am at a loss as to why this would ever happen, any help would be greatly appreciated. Also, after it shuts down and I restart it the time it stays running decreases and decreases. But only when windows is running does it do this little trick, never during POST. So I am lead to believe it is a cooling problem, but I've gone as far as to put a box fan up next to it pulling air out of the case, but it does nothing.

Again
Thank You
Digiman1134