Odd Issue

I have a minor problem and I am not sure what to check specifically. What happends is when I turn on my PC (sometimes it does it when you don’t shut your pc down correctly and it runs checkdisk in windows 2000 pro) after a few hours or so of use (as I said its all random sometimes a few seconds sometime 3 days) the monitor goes blank as if it lost the signal from the video card, my CD Burner and Master Hard Drive, both the lights turn solid and nothing happends you have to reboot. I checked the Video card and its fine I even replaced it with a known good one so I am really clueless to what is causing this but I need to get it fixed asap. Any help is appriciated.