Dear @tim_f,
Glad it worked. But just to complete this thread: the reason the movie player played was because Isadora must tell the movie to start playing before it enters the scene. This is because it can take up to 100 mS for the movie to deliver the first frame after you tell it to play. Therefore, to have the first frame ready by the time the scene has actually become active, Isadora tells the movie to play at the end of it's "preload procedure", which is done when you Jump into the scene. The reason it worked some times and not others is unfortunately somewhat variable, which is an inherent problem in both AVFoundation and QuickTime. In other words, sometimes your zero speed go to the movie before that first frame could actually be delivered.
Hopefully that allows you to make some sense, and is also to document the fact that this behavior is not a bug but just an inconsistency that we have to live with.
Best Wishes,
Mark