My guess is that something is set to shutdown AFTER a particular task runs. For example, many programs have an option to shutdown after a download. I think uTorrent has this option. There may be a automat shutdown after windows updates too. And virus detection programs may have an auto shutdown option that kicks in.
Suggestion... run Task Scheduler Manager and turn them all off and see if it still wants to shutdown after you reboot and let it run a while. Even easier is to google for a program called Autoruns and it lets you uncheck boxes to disable scheduled tasks. That's much easier that the windows method. While you have it.. see what else is loading when your system boots up. (Be careful with the services though...