You would think Apple would have it bulletproof, right? Record on the iSight, edit in FCP, no problem….but….
Reading around on Apple’s support sight, the best info I found is that the iSight camera/QT Pro does all kind of trickery to record a decent image, like changing shutter speeds, frame rates, etc, during the recording. And as a result, sometimes the audio and video sync suffers.
That said, I actually found that while the playback of the QT movie (I recorded “native” because when I recorded in h.264, the video was too choppy, like a series of semi-stills) in my FCP viewer window went out of sync over the course of a clip, once I actually chopped up the clip into a timeline (I’m only using short bits) the sync wasn’t as bad.
The best bit of advice I’ve read is to record in “native” in a brightly-lit setting, which makes it easier on the computer. That said, it is really surprising that with the popularity of vlogging that Apple is putting out disfunctional software/hardware!