Interesting! Running the mem test (for just over an hour, no errors) and when I touched a key on the keyboard it rebooted a la the orginal problem.
If you're running an USB keyboard, try unplugging all other USB devices then see if it still does this reboot thing, if it's ok then re-plug the devices one at a time and retest each time, this probably won't cure anything but at least it eliminates possible resource conflicts (I had a printer that prevented a pc from booting if it was plugged in but worked fine when you hot plugged into the USB port.)