I have a old agp 8x board p4c800 deluxe that is starting ok with boot up but then loses the video signal. The cpu is not shutting off. I am able to read the set up numbers for the heating of the cpu and motherboard and those numbers look good. I have tried another card in the agp slot and the same thing is happening. Could I use a motherboard post testing card to figure out what is wrong? I don't seem to have the problem at start up. It comes after about 10 minutes of running. Looking for suggestions. Buy new MB? or diagnose first? I have bought a PSU CORSAIR|CMPSU-650TX 650W RT from new egg thinking that was my problem. but it has not helped.
Assuming P4C800-E Deluxe and as you described this seem to be either a bad MOBO or overheating issue. Clean the CPU Fan heatsink with compressed air and/or a pipe cleaner; yes I read the temp comments. However, they're not always right and once expansion occurs from heat lots of items can fail in a 'heated' state.
Your MOBO is getting old and often when heat builds up any one of many chipsets, MOSFET, capacitors, etc they can fail. You ruled-out the GPU, PSU which leaves the MOBO, CPU <or> peripherals as the culprit. Have you ruled-out your monitor?
My money is on the MOBO.
edit: Cannot hurt - try the Clear CMOS jumper procedure ; see manual.