Have you tried rendering the movie? If so and it still happens, then clear your temp space and re-render. Also make sure that your hard drive is defragged>

Good luck, and do post back if the problem persists or youe need more detailed instruction.

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IBM A51p with 160Gb Boot Disk, 160Gb external Firewire Disk, 2Gb Memory and a super duper sound card, 17\" TFT Monitor etc etc etc
Sony TRV33e : Sony TRV310e : Canon XM2
ULead products mostly including Mediastudio Pro (V8.0), Video Studio 8, Movie Factory 2.0, Premiere Elements

Remember, there is always more to learn than there is to teach.