So a client gave us a Quicktime video file that was fairly large (150 mb) but not huge. When I went to play it back, it would stop and kind of stutter half way through. I figured it got corrupted, so I had the client re-upload it…and the new one was doing the same thing. On a whim, I exported it as a new Quicktime movie. Even though this new file ended up much larger (420 mb) it plays fine now. Strange.