I've been using Sony Vegas 7.0 for a while and I've never had trouble with any playback before. When I put up the video I'm editing it has trouble with letting me preview the video. It tends to jump and move by freeze frame and it isn't smooth playback at all. I've tested other videos and they work fine, they all playback but this set of videos that I'm editing don't seem to work.
Videos are all AVI and the audio formats are MPEG Layer-3, however all my other videos are the same and they tend to work just fine.