Let me take a stab at this. The tach runs off the alternator. Are the alternator working and voltage regulator functioning properly? I would check the connections on both of these. The assembly at the factories is less than ideal. I have had issues with these that are repaired by proper stripping and crimping. If everything checks out I would pull the panel in the cockpit and check those connections. Not sure how to check if the tach is functional, but someone will. My bet is that with all of the filter/fluid changes something was bumped or pulled and is making a poor connection,