I'm using an unlinked video track with an unlinked audio track. The reason I say that is that exporting was working fine for me until I unlinked the audio, did an edit in an external app (express.adobe.com), and then added that edited video back to the timeline.
The export progresses just fine until about a third to half-way done, and then slows to a crawl or even STOPS.
This is an export that shouldtake about an hour - but I let it go overnight and it was onlyat about 50% with zero noticeable additional progress.
I don't know if this has anything to do with the unlinking/editing, but it's the onlychange I made (I think).