This has been resolved in Pro Tools 5.1With a large session (40-64 tracks) and 2 or more sends per track going to a bus, the video playback will get jerky and lose frames at approximately 3 minutes into the session. It can be related to the use of the sends on the mixer or could also happen with plugins using lots of DSP. The workaround is to remove the sends from the tracks. After doing this the movie will playback fine.