So I recently found a Compaq Deskpro EP (according to the BIOS, the front panel model part was ripped off) at a thrift store for $10 missing the RAM and the HDD. I put RAM in it, and it does POST. However, after about 15 to 20 seconds after booting, the screen will go blank (monitor going to sleep) and the power LED turning red and blinking (this is using onboard video). I've read online this seems to be a PSU problem, so after checking the mobo just took a standard ATX PSU, I tried two different known good ones. Same result. Used different memory sticks. Same result. Unplugged everything but motherboard, same thing. Tried a PCI video card (no AGP slot), and it actually stayed on. despite the power light blinking. It seems it disables onboard video and blinks but doesn't turn off.

Anyone know why this would happen? I am a bit worried.

Specs:

Pentium III 600EB (133 MHz, 256 KB L2)
128 MB PC100 SDRAM (added)
PCI GeForce4 MX440 64 MB (added)
TriGem SpinPoint 15.3GB HDD (added)