What did you check on the belt?
Check that the belt is not too loose or too tight, and also check the rear of the belt to ensure it is not worn.
Try adding some belt lubricant if you have not already done so.
Reply back and let me know if that helps?
If the belt is good, then the motor itself is suspect.
You can test the motor using an independent power supply - if the motor does not run correctly then it is faulty and will need to be replaced.