I've encountered this with several movies that I've... uhh ... "borrowed". One in particular caught my attention, because I lost my broadband with 26meg left in the second disc. Since I was going to be without net for a while, I watched it anyway. I had 2 substantial "white-space-gaps" but sound was unaffected. Fast forwarding to the end of the whitespace got it back, if I played thru the gap, I'd lose lipsync.
Recently I got dialup until broadband arrives, and the whitespace in the clips corresponds to the remaining segments of the file to be downloaded yet.
Basically, what you're looking at is corrupted data in the movie. It essentially means the same thing, except in a more permanent fashion of what I encountered.