What makes iMovie 11 crash when exporting a project?
You can try to convert the defunct video to a different codec. Use something Lossless like ProRes, if you can, or Motion JPEG. Then put the clip back into your project. 

I had this problem once in FCP7 for about a week. It didn't go away until I re-recorded that video (which was painful, but in my case, doable). I wouldn't suggest such a radical solution in your case though. There's a lot you can still try. Read the linked post. Basically what I did was: "circumvent or force media conversion at different production stages".

You can also open Console and take a look at the log when the app is crashing. That normally gives hints what is going on in the background.