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Trashing iMovie's preference file will often fix problems.
Go to your User (Home) folder Library/Preferences (not the Library folder at the top level in Macintosh HD). If you are on Lion (Mac OS X 10.7) or Mountain Lion (10.8), the User Library folder is hidden. To access it, click on Go in the Finder menu bar, then press the Option key - the Library folder will appear in the drop down list.
In the Library/Preferences folder, locate a file named "com.apple.iMovieApp.plist". With iMovie closed, drag this file to the Desktop. When next opened, iMovie will create a fresh preference file with the default settings. If iMovie now runs OK, you can change these settings in the menu item "IMovie>Preferences" if desired. Also, it's now safe to drag the old preference file to the Trash. Trashing the file will not harm your existing Events or Projects.
Does your issue still persist ? If not, Could you please provide rating to me. If you have any questions please let me know.