Peter,
In my opinion the simultaneous existence of the motion problem and the chuff problem points to a poor relationship between the tachometer tape (black and white stripes on the flywheel), and the electronic detector that reads it.
Check to see if the detector is positioned closely enough to the tape on the flywheel. If I recall it should be about 1.0 mm. It's likely to be too far away presently, thus causing both problems at the same time.
Mike
