I just had this “hair pulling” problem trying to output a HDV show. I discovered that some comps I built in After Effects in the HDV codec were causing the problem. They imported fine into FCP and did not require rendering. I rebuilt those comps with the Animation codec, imported them to FCP and rendered them in the timeline. Then output work like a champ. This might not be your problem, but if you’ve got some After Effect comps, try pulling those out of the timeline and see if it works. Hope this helps.