What spec is your computer and is it powerful enough to meet the requirements needed by PD? How did you transfer the footage onto your computer? When was the last time you defragmented your hard disk? If you haven't done it in a while then do it now and see if that makes an improvement. Also switch off or disable Anti Virus software and screensavers as they can affect some software when it is running.

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Better than a poke in the eye with a pointy stick any day...