It probably not a tach if it moves ok in rapid, Tree's will error out first during rapid. Get the drive tuning procedure from Bob at ZPS. You will need to pull each card out of the drive rack and use a DVM to set pots. Then set the axis lag using the set up page, and set them for 15 at medium jog. The set up sheet has all the details on it. I've also had the large capacitor on the drive cause this, they go bad and no longer can control the ripple on the drives.