Okay, it sounds like there is some corruption in your source video or project. What is the format of source video you are using ? Is it MOV ? If it is in another format like AVI or MPEG or MP4 first convert the video to MOV. You can do it with the help of Quicktime. What you need to do is open up the video in Quicktime and Export it with Quicktime as a .mov file. Then import that mov file into iMovie. Make the edits and try to export again.
If you are using a mov format video and it is crashing surely it is because of an errored project or source video. Sometimes your source video may corrupted sometimes a transition or title in the project may the cause.
A lot of people complaining about Organic Main Title crashes their project. So if you are using that on your project change it and try the export again.
If it not helps, Quit iMovie; go to Computer > Movies and move the iMovie Projects folder to the desktop. Open iMovie; create a new test project using film footage from the Event Library. Try to export it. If iMovie does not crash, the problem is being caused by a corrupted movie project not because of that video file itself. You need to create a new project and try exporting again. (If it crashing, the problem is with the source video. You need to make your file piece by piece, and use those video pieces on a new project.)
If it not helps. try the suggestions in this discussion
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