While searching for help with a FCP issue, I found this thread. I know these posts are old, and they refer to older versions of FCP & QT, but I'm having the exact same problem with newer versions of each. I'm running FC Studio 2 and QT 7.6.4.
I had the same problem with a DV footage capture. Still haven't determined if it's my camera or what, but the symptoms are exactly as described. I wonder if it's a QT issue a well.
Most likely a QT <> FCP interop issue, especially if you've done a QT update some time before the problem appeared. Not in a position to try to test or recreate the issue since thanks to the bomb, bailout and bankrupt republeconomy I'm having to stretch as much life out of my PPC Macs and FCP 5 as I can. You might try direct capture with QT itself to see if the problem persists there.
Best you can do is report the issue to Apple and pray to the cosmos that they do something to address it. I never saw any admissions of error or notifications when the problem was resolved the last time - just the usual customer disservice from the corporate welfare state.