From your brief description this sounds supsiciously like it's an interlacing issue.

This has been discussed many times but in short....

You have likely recorded an interlaced video clip on your camera and imported into your project. no problem there. Yet you are attempting to view it on a NON_interlaced monitor. Hence the jagged edge on movement.

You can either choose to de-interlace the footage if you are ultimately going to view it on a computer monitor or you can simply leave it and export your final movie as interlaced and build a DVD to watch on your TV where it will look fine - unless you get the field order wrong like I did recently. D'oh!

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