You're using the webcam and screen grabber to capture DV video? Probably not the best way to do it.
Is your miniDV camcorder attached to your computer via a FireWire connection? If so, there's a tool in PowerDirector for copying the video from your camcorder.
Are you using a third-party device to connect your miniDV camcorder via USB? There's still a better way to capture your video than using the screen grabber, and using it will likely remedy your issue.