Random restarts with Leopard

I may have discovered an undocumented new feature in Leopard.

I'm using a new (Sept 2007) 24 inch iMac. A problem that seems to have come up with my install of Leopard in late October is that my computer tries to restart itself after a period of inactivity. The computer can either be sleeping or running with the screen saver on and it will just restart.

If I have an unsaved document in Word, Excel etc... opened or browsing with multiple tabs in Safari or Firefox, this will prevent the restart. Instead, I'll have a message on the screen saving "are you sure you want to quit Word or Excel without saving" or "Are you sure you want to close multiple tabs". I didn't initiate the restart, this is something the Mac did on its own after not being used for an hour or so.

I don't have a scheduled restart or shutdown set in my preferences and I don't think I see anything strange going on in my console logs.

The temperature of my PC always remains between 40-50 degrees according to iStat - I originally thought this was a temperature thing...

Any ideas what it could be? Or some script I can look for as the culprit? Could this be just a bad upgrade to Leopard or something else?