Comment 11 for bug 397839

This sounds like a problem with the xserver dpms timer or gpm or whatever you are using to control the dpms timeout. If you see things like:
disable LVDS
enable LVDS
in your log when it happens, then some external event is causing the driver's dpms function to be called. Some other possible causes include video apps that fiddle with the dpms state to keep the screensaver from kicking in, or in the case of a momentary blanking of the screen on older cards, apps that query the randr state on a regular basis.