I'll bet this one addresses your issue.
http://www.camaros.net/techref/electrical/tachfilter/
I built my first tach filter over 20 years ago, when my 5" autometer tach would not work right with my new HEI.
Six years ago, I had to remember how all over again, when my new repro tictoc tach wouldn't work right with my electronic ignition.
The above post has been part of this site since. It's in the electrical basics sticky at the toip of the electrical forum. I've built and mailed dozens, and parheps hundreds of others have built thier own. (no, I don't build them for others anymore) THey work.
Tach filters are also sold by a number of ignition manufacturers, including MSD. Similar devices were used on lots of production cars, and they can be found in a junkyard if you know what to look for (note that I don't, so don't ask me)
Tachs are frequency counters. The pulses from the ignition system are converted to a DC voltage that is put thru a coil to generate a magnetic field. The strength of the field is proportional to the number of pulses. The condensor in a points type ignition is almost a tach filter. Electronic ignitions tend to put out a lot of high frequency noise, which can be seen by the tach as extra pulses. Various other things can happen to the signal as well that will "confuse" the tach.
Since electricity will always take the path of least resistance, the filter uses a capacitor and a resister. The resster feeds the tach and provides a constant (small) voltage drop. The capacitor's resistance (actually impedance is the proper word) varies depending on the frequency of the signal. To a low frequency, it is a very high resistance, so the signal passes thru the resister to the tach. Higher frequency's see the capaicotor as a low resistance, and seek ground thru the capacitor, bypassing the tach.
The filter I designed cuts off around 10,000 rpm. If you rev that high, your tach will go to zero!