Weird Mac Problem

Has any one else seen this problem? Using a MacPro with Snow Leopard 10.6.1 on it, and it seems like everytime I go to shutdown the Mac (using Finder and holding Alt and clicking on shutdown) it shuts down, but then a second later it starts back up. (almost as if it was doing a reboot instead of a shutdown. When the login screen shows up, I click the shutdown and it shuts down and doesn’t restart. Any suggestions?