Bouncing Tachometer in idle. No engine revving

Had new starter assembly and new alternator installed today. Drove a block away from the garage and the tachometer went crazy. We had no issue with the Tachometer prior to this installation. Mechanics tell us there was no problem after they tested the car. Tachometer bounces back and forth with the needle going ultimately off scale while in idle and while driving with no engine irregularity. What could be causing this problem? Took car immediately back to the servicing garage and the mechanics tell us we have to leave the car for further testing because they can't figure what's causing the problem.