Stuart: I am just speculating here as I don't have all the information I would like. It sounds to me like a graphics issue, either your card driver being out-of-date, old version of QuickTime, or something along those lines.
Also, it might be that the clip is of a different format or is somehow corrupt in terms of the video and that might be the cause of your problems. It is really hard to say without a snapshot of your workspace in PD and also supplying a dxdiag file (instructions in one of the sticky threads at the beginning of the forum).
Sorry that I can't be of more help, but without more information, we would all be throwing darts. Another possibility is to turn off hardware acceleration, if you have it turned on, re-render and see if that makes any difference.
Have a great day.
Regards,
-Phil
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Bleeping Computer Malware Response Instructor