Sounds like a faulty motor starter if equipped or motor it self faulty.
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if the problem reoccurs after the calibration then you may need to replace the speed sensor... it is usually located next to the drive motor pulley (it is a magnetic pulse speed senor so it should be facing the magnet on the motor pulley) P.S. if the sensor is not lined up this can be the cause of the problem also, so check that it is before replacing.