dave,
I think I have experienced what happened to you once. It was maybe 1.5 to 2 months ago, I got on it a little hard when it was cold out, and the engine was cold. It seemed to rev a bit going into second or third (not sure which one, or if sport mode was on) without the gear really catching. I backed off and everything was fine. Now I wait until at least two bars on the oil temp is reached before driving hard, and it has never happened again. If it seems to be happening constantly during easy shifting, I would bring it to the dealer.