I have a Compaq Presario 6000 PC with a P4 2.53 ghz processor that rendomly started booting and dying all on its own the other day. I work on my laptop and this machine is used by the wife for internet and games.
I opened the case and started to look at things. The first thing I looked at was the power switch, which when connected to the board while the box is plugged in will result in random start/stop activity - even without user intervention. I got the machine totaly booted and shut it down, powered off and unplugged. I disconnected the power switch from the board and plugged the machine back in, and the behavior stopped. Now here's where I screwed up (I'm a half assed hardware tech, I break more than my fair share of stuff this way) but I plugged the switch back in while the power was still plugged in and it fired up, but my snap reaction was to pull the switch connector again (You plug something in and something whirs, sometimes you justundo what you did.) Strangely, the machine booted up the first time, all the way, and stayed on (which it hasn't done for a while with the switch plugged in). So, knowing that I could short something out, I shut down the box, unplugged the power from the PSU, and plugged the switch back in. I plug the box back in and without pressing the power button it starts to switch on again. It goes through a cycle of off and on and never actually boots up all the way, finally it shuts off and I pull the plug just as it's coming back up (This is a very rapid boot/stop/boot/stop sequence, sometimes it powers up all the way to the windows loading screen, sometimes it doesn't make it past the bios setup screen). I'm thinking that this is either a short in the switch, or a short in the board where the switch is connected.