Yeah... I have the same problem but not with all the DVDs, and so many people have noticed the same problem..
Maybe an update through xbox live would solve it...
Yeah, that's something I forgot to tell you.
If you have HDTV no problem, but if you have an standard TV you should have selected PAL-50, at least is what people say, try it and tell us.
( but the problem is still appearing with some dvds)