imovie '11 why do some of my frames export as solid black frames?

So ive created a movie in iMovie 11. The movie plays perfectly fine in imovie, but ive tried exporting it several times via quicktime and each time it exports with certain frames blacked out. I also just tried uploading it straight to Youtube and the same thing happens. heres what the movie lookes like exported:

http://www.youtube.com/watch?v=n8JrIvjLigA

can anyone help me figure out how to fix this? fair warning, im not the most computer savy.......

thanks for any and all advice!