yes, one newbie here giving advice to another. I am sure I am more of a 'newbie' than you. It seems like if the tach is the typical analog type you might try using a magnet to bring the needle back to 0. If yours is digital I have no thoughts. the analog tach's and speedo's in many vehicles use voltage fluctuations to move the needle by induction. makes me think that maybe there is a damping system that is stuck. the damper takes the bounce out of sudden voltage fluctuations. try a magnet, it can't hurt. an old speaker magnet may be needed if a smaller one won't. sorry, that is all I got. good luck.