> Some of the clips have a section at the end with audio only (no video), even though there is
> video and audio on the tape.
> I checked the source file QT, and it plays fine....
Now that's bizarre. It's not a missing codec if QuickTime Player can play it.
Try opening the clip in QuickTime Player, put In/Out points covering only the missing area, Copy/Paste into a brand-new QuickTime Player file. Save under a new name (related but not the same as the old name), import the file and check. I think there may be a "digital spike" in the file that causes FCP to conk out at the same point every time. If this doesn't work, try the method again and shift the In point forward, losing some footage, but you may be able to salvage the rest.
You can also try relogging that section of tape, again, to have it cover less of the affected area each time.
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