will it move with high rpm's? It kinda sounds like a spun propeller where sometimes it will move at very low speeds but when you increase the rpm's the shaft spins because the hard rubber material inside the propeller is shot. This is a feature designed to protect your motor from damage as replacing a 200.00 propeller is much cheaper than lower end work.
I hope this is your problem because it is a easy and inexpensive one to fix.
Greg
September 2009