Is this a new powertap? It sounds like the harness might be messed up. You don't have any zip-ties on the wire portions of the harness do you?
How close is the shark fin to the hub?
Are you certain the head unit is completely set in the mount? (solid-feeling click)
Regarding data from the race, it will be pretty much useless from an evaluation point of view. If it's around an hour, and you race really really hard, you might come out of it with a ballpark +/-10W FTP guess.