OK, if you're sure it's not a thermal issue.
First, see
http://support.microsoft.com/kb/555541 - to enable the Shutdown Event Tracker.
Look at a few other articles like
http://technet.microsoft.com/en-us/l...8WS.10%29.aspx and
https://www.microsoft.com/resources/....mspx?mfr=true
See if that provides a clue, the next time it shuts down.
I would suggest drastically reducing the frame rate, to see if that helps -- but I can't think how to do that in your situation. Perhaps somebody else can think of a way.