I'm having/had the same exact problem. I was able to clear it up using locV and locH (I tried one or the other, then when it didn't work I moved the .mov even further offstage at first). It seems to work fine, except I still get the flash occasionally. When I build and test a projector though, it goes away. At least, it hasn't appeared in a projector yet. And I have a transition in the movie. Still, it'd be nice to have a clear and solid fix for this. Anybody know what causes it? Is it possible that we're seeing part of the buffering process?