When I purchased my 81 300sd 6 months ago the dealer installed a new tach amplifier. Every so often get the needle would bounce a bit but alway stabilize. It now will not move unless the engine is under high throttle. When I let off the throttle it stops working again. I recently put in a new alternator and voltage regulator. What else could be causing my tach issues? It seems power related? Thanks in advance for any help you can provide.