I added an original '67 gauge set from a donor car a few years back to my car. When I installed the tach, it seemed to behave very similar to what you describe. Nice and steady at idle, but would swing at a much higher rate than it should have given the engine speed. It was not erratic, just too high. My car was not delivered with a radio, so there were no filters. So I added a homemade RC filter at the coil connection and fixed the problem.