iMovie HD Beachball Problem


I'm starting to get annoyed at this. Every time I do anything in iMovie that brings up a progress bar, importing, saving, compressing, the program immediately brings up the beachball. I can freely change to iMovie, but can't do anything. The progress bar dosn't move, when importing it says on "Preparing". But if I leave it long enough, whatever I was doing will be completed and iMovie will work fine.
This has only just started happening. Any ideas what might be causing it? I know it's not that serious, but I'd rather it didn't happen.