Erratic tachometers can be from a number of culprits. The tachometer operates by reading a "pulsed" signal from the ignition coil from the negative terminal. The AC sinewave (thereabouts) is read and the tach reacts. The old style (like yours) operates on this high voltage signal, newer tachs will read a low voltage signal (an old tach will not work on an HEI with a hall effect signal. )
Enough about that....
Several factors can make a tach erratic may of them would require an oscilloscope to verify but here are some places to start:
- 31 culprit is bad ignition wires/distributor cap, this can "bleed" a signal to the tach source wirte on the negative side of the coil (tach output on older HEIs)
- Coil going bad or if the tach has a ballast resistor that might be going bad
- The older tachs usually had a silicone dampening fluid in them, that may have dried out and the tach is operating properly but without the dampening fluid the pointer looks like it is moving erratically.
- #2 reason: The capacitor(s) in the tach are going bad. They are just a battery that charges and discharges quickly. Just liek a battery they have a definite life.
Sometimes a tach filter will clean things up and keep it working well, but that is usually a short term (6 months-5 years).
If you have an oscilloscope, best way to check for a clean tach signal is at the back of the cluster, that way you can "see" what the tach sees. Then if the signal is good you can move on down the line, without a good signal the tach will not operate properly.
Hopefully this helps