Why won't iMovie '11 export properly?

Clips with a cutaway either export just as the cutaway in 720p HD (even though its set at 29%), not at all (error -108 with 1080p HD) or with the last clip as black (large video size).

When I play it in the project it works 100% fine.

No idea what's going on...

Can someone please help? I know it has to do with RAM but seriously this video is 34 seconds long, I have no other apps running at the same time, nothing else going on, yet it still has no RAM? How do I clear up more RAM?

Super frustrated...

Thanks!

Natalie