Full screen messing things up

Hopefully someone can help me with this issue. I was making a video in movie maker and had the timing going good (it's an amv) until i got to a certain part in the video where the timing went off for no reason when ever i watched it in full screen. It would do just fine in regular mode but if i went to preview it it would be off unless i edited it to make it off in the editing screen. The beginning always did fine it was that part and a couple others that would do it for no reason.