Imovie clips quit working

I need some help here. I created an Imovie '11 using a macbook pro 10.7.5, and I created the entire film on the macbookpro so nothing was imported. Here is the problem. WHen I scrub through the film with the mouse, everything is great, everything works, but when I hit play, some transitions quit working and some of my clips are just a black background (no picture) (or even sometimes the wrong picture will appear).

What can I do to try and fix this.