Bad CPU or bad motherboard?

[rationull]

The other day, during a period of especially heavy processor load I lost video signal. The reset button wouldn't work, so I just gave it a hard shutdown, and after that the power button wouldn't do anything. After a short wait, when I pressed the power button again the machine would come to life for about a quarter of a second and then shut down. At this point, the machine turns on, giving power to the fans for a few seconds before it dies. It will stay on longer the longer I wait between power cycles.

With another known good power supply plugged in the behavior is exactly the same.

With my CPU power unplugged, or with the CPU plugged in but the graphics card removed from the AGP slot, the machine will stay powered up (all the fans are running) but will not access the drives or anything. With a much older, weaker graphics card plugged in (I think it's a Riva TNT), it will stay on and not die (that is, the fans are running. Still no video signal or drive access).

I'm not getting any BIOS beep error codes.

So, I've ruled out the power supply as another behaves the same way. I've ruled out my video card as it still works in another machine. At this point I'm pretty much assuming it's the processor or the motherboard, but have no idea how to diagnose between the two. My gut feeling is it's the motherboard, but since I don't have that much experience troubleshooting hardware, I thought I'd ask more knowledgable people what they thought before doing the RMA dance. Does this sound like it could be a CPU problem?

CPU: P4 Prescott 3.0 GHz
Motherboard: Gigabyte P4 Titan w/ Intel 875 chipset
Graphics: ATI Radeon 9800 Pro
RAM: 1 GB of Crucial DDR 400 (2x512)

 

[RBBRMADE]

Originally posted by: rationull
It will stay on longer the longer I wait between power cycles.

Sounds like a possible overheating issue.
Make sure your heatsink is installed firmly with good thermal paste.
Make sure your fan on the CPU is spinning and plugged into the correct header.

Ron

 

[rationull]

I would think it was a heating issue too except for a few things:
1) It's worked fine for the past year.
2) It continues to run when my graphics card is not plugged in, but will not access drives.
3) It gives no BIOS error beeps when my graphics card is in, but its direct power cable is unplugged. (another machine with my graphics card in it does give beeps in this situation)