Possibly something in this thread will help:
https://discussions.apple.com/thread/3719997?start=15&tstart=0
In that thread, particularly check out the suggestion by DryFlyRy which appears to be the solution that worked for most people there.
A couple of suggestions from that thread -
Look in your project for any effects you may have used, such as speed changes or reverse motion, or perhaps several copies of the same clip. Try removing any suspect effects or clips.
Also, duplicate the project and see if the duplicate exports OK. To duplicate a project, in Project Library view (where all your projects are listed) click on the project then from the menu select File > Duplicate Project. The duplicate will have the same name as the original, but with a number appended. The name can be changed if desired.
John
Message was edited by: John Cogdell