Not so much an answer....
I have exactly the same problem. Reebok Fusion Treadmill by any chance?
I have seen a lot of similar complaints on various web forums.
People are recommending that you look to get the motor repaired (as technically it is just another motor and a mechanic should be able to fix it).
I found these guys.....
http://northwick.eu/treadmill.aspx
They do call outs, repairs, spare parts and are licensed agents for Reebok Fitness. They should be able to help you out.
No good for me though as I'm in Northern Ireland and they are mainland UK only.
If you get the problem sorted can you post the solution on here.
Good luck!
Hegz 1981
January 2010