I need help figuring out if my unit is bust or I have it set up incorrectly! It's a joint Video/DVD theater system and the video plays fine through external AV input. But the DVD goes to a black screen and only plays sound. When I fiddle with the TV menu controls to alter the external input selection and then change it back, the image briefly appears on screen ( for a split second), then goes to black screen again. Any ideas what's causing this? The unit is only 3 months old so still under warranty.