DVD Player 4.6.5 won't stay Full Screen

In my classroom I frequently play movies through my iMac (OS 10.4.11) using DVD Player 4.6.5 hooked up by VGA to a video projector. Everything works great, except for one persistent, mysterious problem. I show the movies in full screen, but EXACTLY every 11 minutes (11:00) it pops out of full screen, back to whatever smaller setting it was in last (half, normal, or maximum). I then put it back to full screen, and 11:00 later it happens again. It makes no difference whether or not I select "Remain in full screen when DVD Player is inactive" in DVD Player Preferences. Also, it has nothing to do with screen saver or energy saver functions in System Preferences. Furthermore, it has nothing to do with whether DVD Player remains active or not. Let's say it pops out of full screen at 10:42:25 am. Let's say a few minutes later I eject the disk and quit DVD player. When I put a disk back in and DVD Player restarts, and I put it back into full screen, it will pop out of full screen again at 10:53:25 am--exactly 11 minutes since the last time. Obviously, this is really annoying when trying to show a movie for an hour. Is it something in the system software that's sending a signal every 11 minutes? Or something in DVD Player that syncs up to clock time and does mischief? Many tech people I have consulted at work on this one have dug through my software and OS and shrugged their shoulders.