iMovie Photo Slideshow -- photos `flicker' when played

I have been working on a photo slideshow in iMovie '08 for a couple of days. Everything has been fine, but all of a sudden today, nearly a quarter of the photos seem to flicker or jump when they come up during the show. The `jump' only lasts a fraction of a second and then goes into the Ken Burns effect I set up.

I have it set up for each photo to be on screen for 3 seconds. Any ideas or thoughts on this would be appreciated.