If you are reasonably competent with a small soldering iron it may be worth opening up the Tacho to see about repairing it.
Is the engine completely smooth running? I ask this in case the jumping about is due to a poor connection between tacho and coil?
I vaguely remember something about the electronics inside the tacho. The input signal operates a transistor which then charges up a capacitor. The capacitors do not last indefinitely and can become erratic with an internal short. I recall finding this on a google search. Replacing the dud capacitor restores the normal tacho function.