Hi, I'm Pete. Welcome to JustAnswer! I'm reviewing your question now, and will post back with your reply ASAP.
First, check that the speed sensor is correctly adjusted - try measuring the voltage on the wire from it when the motor is turned and let me know if you get a change in the reading?
If the speed sensor is good, then suspect a problem with the motor.
Is there a buildup of carbon around/under the motor, or is there a burning smell from it?
Can you test the current draw on the motor when it is running and let me know what it is reading?