Stuttering Video In Imovie

I am working on a 45 minute movie in imovie 4 and as it gets longer (after 35 minutes) the video is stuttering and stopping on playback. The audio usually proceeds along fine. I have 150 gigs available on my external hard drive so I shouldn't be running out of space. Any suggestions to smooth out my video playback.
I sometimes wonder this, as quicktime has quit on me on a large file. Then I sort of came to a conclusion that I may have the screen resolution to high, so reducing that It's never been a problem since....