First, make sure your project is saved and then quit iMovie and do a cold-start of your computer. iMovie deals with so much raw data that sometimes (in my experience anyway...) it just gets a little confused.
Second, ensure your're not at or near your HD capacity. iMovie can invisibly use gigabytes of swap pages, and the closer you are to filling your drive -- the more fragmented the available space becomes, resulting in lots of little writes instead of just a few big ones.
Third, what is your HD speed? iMovie definitely prefers at least 7200 rpm drives; if you're using it with a laptop (4200/5400 rpm) you may experience intermittent problems. And on that note, if you are using an external HD I would recommend connecting via FireWire vs. USB-2.
Fourth, what audio bit rate did you use for your original camera recording? Some cameras default to 12-bit audio, which is a no-no as it can cause audio sync problems. Stick to 16-bit and you'll be fine.
Good luck, hope this helps!
i'm close to finishing a 50gb imovie 6 project and have suddenly experience a general audio muffled/garbled problem throughout project. I was wondering if i dump the video clips pane will help solve this problem. I've prob. got like 20 min.of footage left in the clips pane. trying to solve this problem b4 working on the project again. Any tips or comments greatly appreciated.