I had the same problem here on Xubuntu 12.10 x64. I traced it down to DPMS (some kind of TV out power save feature I think....) and following this thread
I did this
I then set that line to run on startup and set all and every power off/suspend/screensaver to 2 hrs. That way users can watch a movie. Any other setting and users were in the 10 minute zone - there shud be another way but I couldn't find it.
xset s off && xset dpms force on && xset -dpms &&