Q: imovie projects freezing

Have an older IMac desktop, but still seems to work fine. Just completed a movie in IMovie (have done this over a dozen times) and it has froze (bloody little ball just keeps spinning!)  The event is on an external hardrive to save space on my desktop. Probably added too many transitions and would like to go in to the project and delete most if not all so I can complete and make a dvd - as I have done before.  Any suggestions?