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What is the specification of your Macbook?
Have you tried playing back on another Macbook?
Usually this problem is due to insufficient resources being available on the computer.
Your iMac meets the spec required for iMovie 11.
Do you have any other apps running in the background? If yes, try closing those and let me know if there is any improvement.
Also check in Activity Monitor to see what processes are running and how much CPU % they are using - send me a snapshot when the problem occurs if possible.
Ok, all appears fine with the CPU.
I will open this question up to the other experts to see if they have any further ideas.
the other experts are still reviewing your question.