Interesting issue with a new Optiplex 960: whenever the machine is shut down, or even put into S3 sleep, the machine restarts itself 4-5 seconds after powering off. I figure it can't be an OS issue because, well, it's off! I did reinstall the BIOS (can't call it an update since it was the same/latest version). I find lots of references online to machines not shutting down, but none to a machine starting back up after complete power off. Only way I've found to work around it is to unplug the power cord after shutting down. I'd love to hear some thoughts on it!
I had a similar problem with my last computer. Whenever I put it in standby, it would stay in standby for a few seconds and then turn itself back on. It turned out to be a problem with the internal (C:) drive. There was some intermittant trouble in the electronics of the drive. Eventually the drive, literally, blew up! You might be looking at some early trouble in your drive.
I have a GX620 that does the exact same thing. With a power-down or hibernate, when the system goes down, the green power-switch lamp goes off, then a second later, the system powers back up on its own. When it first started, I truly believed it had to be related to the motherboard, so before the system went out of warantee, I managed to get Dell to replace the motherboard... No joy, still does it.. I'm living with it, by holding the power button down to force a power off. Could it be the front panel board or ribbon-cable feeding the front-panel board??? Love to get this fixed...
Further note: this can't be an OS problem as I see it under XP, Vista, Win7RC, and Ubuntu (this machine has a flock of harddrives I swap in/out for the various OS)
Note to original poster: if you hold the power button down for 10 sec or so, in my case, the system does power off and stay off...
We seem to have solved this now by removing the CMOS battery for a few seconds then reinserting it.
When the machine is powered off, it stays off.