OK I was working on my PC, and since it's on the floor, I accidentally bumped the front panel a bit. Not extremely hard, but there was a bit of a *thud* sound. However, it reset itself. Then I tried running things again and a minute later it reset again. So I unplugged it for a while, plugged it back in, pressed the on button and... nothing. I pulled it out and used another power cable, it came up without problems. I tried the original cable, no problems. I pushed it back under my desk, and... nothing. Unplugfged it, plugged it back in, and it came right up! WTF?? It's not a heat issue, as it was in that position for a week now without problems, up until I bumped the front case cover. (besides I have a fan on the back that came from some Sisco network rack-mount routers. It pushes roughly 38 cubic feet of air per minute, but it's noisy at 41 decibels!

so there should be no heat issues.) I bumped it once before and it reset, but it was stable since then until this happened. Well, except for when I pulle dit forward to plug in an audio cable, it reset then. I need to re-work my desk arrangement, or be more careful, I guess.
But if I leave it alone when it's running there are absolutely no problems!
Any ideas as to what could be causing this?