My xbox just started turning off tonight.
It started right after we finished a round of Halo 2.
We finished and the box shut off.
When I tried to turn it back on, it got to the flubber and died.
Then it didn't get to the flubber.
So I searched, and came across a few threads about problems and possible solutions. I found one that mentioned cold solder joints. It sounded possible since I have a few internal mods that require soldering.
So I checked into it. My power supply was ok, figured I would check that since I had it out. I did a remote turn on with a doorbell by following one of the tutorials a while back. This was my problem. The connection on the Power/eject assembly had come loose and was shorting on the board. I don't see how the box managed to stay on or off now that I think about it. I don't have my soldering iron here to fix the connection, so I just removed the doorbell, and now it works great. No more problems for me.
My suggestion:
If you have done any internal mods, that require extensive soldering, check for cold or broken joints. It was the problem in at least one situation, it could be the problem for somebody else.