I get that one too.
If you have a lot of hours on your scan motor you'll need to replace it.
I had to clean my encoder strip and the encoder sensor last time I got this and it took care of it.
Unplug the machine, remove the encoder sensor and clean it with alcohol.
Be cautious on the reinstall. If the sensor is touching the strip you'll kill the strip. Make sure you have plenty of clearance.
That should do the trick. If not, look into a new scan motor.