I went out to my car (1983 LE) the other day and my tachometer didn't work. The Haynes Manual has 2 troubleshooting options:

1. No Reading on the Tachometer. or
2. Extremely erratic reading.

Neither of these is completely correct, because sometimes I will go out and start the car and the tach will work fine. Yet other times, I will go out, and it will get stuck at a certain RPM. What should I do now? Thanks a lot.

 

Check your connetion on the trailing coil. Then test the tach itself (involves removing the cluster). Follow the FSM for the procedure.

 

Seems you have the 83 common problem of your tach sticking. My 83 tach sticks when it's colder. During the colder months it sticks every morning until I get the inside of the car warm. Once the inside of the passenger compartment warms up, the problem goes away.
Do a search, there are some ideas as to how to fix it, (light oil, grease, solder joints) but none that have come back and said they fixed it for good.
If you figure it out and permanently fix it, make sure you post the fix please.