Could be a lot of things. To see if it's something you could conceivably fix, systematically reseat everything that you can practically reseat (the personality card Bolle mentioned is a good one to include). There's also no substitute for a careful visual inspection. Blow out the dust and look closely at the motherboard and cards. Mystery smudges? Cracks in traces? Pram battery voltage? Leakage?

Measure the power supply voltages while you're at it. They should be within 5% of nominal. If any are significantly off from that, that's a clue right there.

Try booting with a minimum of parts (RAM, HD, etc). That will help narrow down the possibilities.