Sounds exactly like what my motor did when I spun the prop. There is a rubber insert inside the hub of the prop. It is made to give when you hit something so as not to damage the lower unite/gears. Instead of the prop turning the shaft and rubber insert is turning. I just took mine to a local prop shop and they were able to press in a new hub for me...not very expensive. If you have a spare put it on and I bet you will see the trouble go away.
