Very unstable Imovie 11 on Retina MB Pro

I have a Mbpro Retina 2,3 ghz 8Mb RAM laptop.

I am using Imovie to make a project. The files I use are about 400 clips with a total size of 20gb.

AFter a couple of minutes skimming is not working and after tien more minutes Imovie crashes. The larger my project movie is the bigger the problems are.

I have tried several things. Cleared up chaches, tried starting up in safe mode and so on.

Nothing worked.

Has anyone the same problems? Apple does not seem to have an answer.