[MacDV] Movie wont play

[Kim]

I am running iMovie 4.0.1 with OS10.4.8. I have a movie that I am  
trying to export back to tape. I know the camera connection is  
working. When I try to export or even play the movie within iMovie  
payback is jerky. If I select less than about 20min worth of video it  
plays back fine. As I select more and more video the playback keeps  
degrading. This does not matter where in the movie I select the  
clips. I have deleted the preferences run Macjanitor and repaired  
permissions to no avail. Any suggestions?

ric