Have you ever messed with the advanced video settings in Avalaunch? Check for dry joints on your video encoder chip, and the AV input/output connections pins on the mobo. If you see any, touch them up and give it another try.
Another thing it may be is defective crystals, I think there is only one in the Xbox, and i'm not sure if it is related to video in this case. (I remember back in the day of analouge decoder repairs they were). You could get some electronic freeze spray, and try it on the video encoder chip, the surrounding area, and this crystal, you'd have to have your dvd drive out to test this though, and spray it on these areas once the video goes crappy, if the spray brings back video, then it's a defective component, you can pin-point it, and replace. These tests can be tricky, so try to keep the blasts concentrated and remember the area(s) you've tried. Other than that, I wouldn't know what/how else to test. Good luck.