I wish I could help you with this Dan but this falls outside of my knowledge area. I do notice that you said the choppiness occurred when played back from within Vegas. What format is the video encoded in? Have you tried playback from within an actual media player? If you can provide a list of your initial settings, I will try and find some information to help you mate.
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You could also drag and drop the video file in to GSpot and get some information from there if need be.