GM Tachometers between the late 1960's and early 1990's had a common problem. They fail!
How do you know the problem is the Tachometer and not the distributor or the ignition module?
You can switch out parts and hope it fixes the problem, for now. Or, you can test the Tachometer.
Once you know the problem is the Tachometer, you can replace or repair the correct parts.
We also sell two different types of Tachometer Testers. Here is a demo video on one of those testers in action.
Some common Tachometer and Testing hook-up locations.