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.
COMPAQ something or other with 500Gb Boot Disk, 1Tb external Firewire Disk, 4Gb Memory and a super duper sound card, 19" TFT Monitor etc etc etc
Sony TRV33e : Sony TRV310e : Canon XM2
Sony Vegas Studio editions to v11
Remember, there is always more to learn than there is to teach.