If this doesn't fix it, the next thing you could try is to clear the system cache. Put Fallout 3 in your 360, then load the game.Hold down the A button on the Player 1 controller.start up your 360, then take out the Fallout 3 game disc.A lot of people have reported that clearing the game cache on your 360 will fix it, so that's what I would suggest you try first. Fallout 3 freezing is an unfortunately common problem.
