Yup, I paid the $180 to have my PowerTap G3 hub serviced a couple years ago, though I now forget exactly what the problem was. But "grinding" while coasting rings a bell. I now have about 13,000 miles on that hub, and that was the only service it ever needed, or received.

A serious cyclist (with an expensive power meter) really should have a 2nd set of wheels (or a 2nd bike!), for times like these. Nobody likes to train for, and then miss an event, because of a mechanical issue.