iMovie keeps crashing/freezing while I am trying to make a movie. Haven't struck this before. Any suggestions.

I've used iMovie a lot but never experienced this before. I've been making a short movie comprised of lots of little clips of a sports team in action. IMovie has kept freezing/crashing every few minutes and I have had to use 'Force Quit' to close it down and continue. So frustrating! Any helpful suggestions? Thanks