You may want to try a few repairs first, just to make sure imovie is running properly
Open your finder ( magnifying glass ) type in disk utility, and launch the disk utility application.
On the left side of the disk utility window, click your mac hd, then at the upper right habnd side click first aid, then click first aid again.
Once that completes, go to your user library folder
This will help you get there. http://macdrug.com/access-the-user-library-folder-in-mac-os-x/
Then, once your in the user / library folder.. Go into the preferences folder.
Then, find any files that start with com.apple.imovie and trash them all
Once trashed, restart your mac and then start imovie.. It should be running a bit better. Just to be safe though, Id consider saving your project every 15 minutes till your sure that it isnt going to crash on you.
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