PC Randomly Reboots

I have a Dell gx110 optiplex 733mhz w256MB Ram that will suddenly reboot itself. I can be looking at a doc, surfing the net, or not even be near the pc. I have returned to see it at the login screen, so I know it decided to reboot. It happens everyday lately, sometimes 2 or 3 times. I’ve contacted Dell Support and suggested a power supply issue. They didn’t totally disregard me but want me to run it in DOS mode over the weekend to see if it reboots, then they’ll take action. Has anyone run into this event?