Answer might depend on which camera, iMovie version, video file format, etc., you are using. For example: iMovie '09: Footage recorded in Digital Cinema Mode may cause jittery playback
If you are really using iMovie '11, perhaps you have disabled the stabilization setting in your import dialog?
If nothing from thisApple - Support - Search helps you, give as much additional info as possible you post back including:
• camera, iMovie version, video file format mentioned above,
• have you changed any iMovie import settings,
• how you "converted" your MTS clips to MP4,
• whether the problem remains after restarting your Mac,
• what software you have changed, updated, or added since things worked correctly,
• how much space is "Available" on your startup disk,
• whether the dozens of films you made were also made from the same kind of imported MTS clips.
Message was edited by: EZ Jim
Mac OSX 10.8.5