Do you mean the backlight is flickering or the needle is erratic?

 

 

Other then the gauge itself, check the grounds and the capacitor at the rear of the engine. Just a long shot with these.

 

The signal for the tachometer comes from the coils (not sure if it trailing or leading for an 85). So you need to verify that using a tach connected to the same lead to see if its the signal that is bad or the tach in the car. If the handheld tach shows the same erratic behavior then you may have a connection issue at the dizzy or a bad igniter for whatever coil supplies the signal. You swap the igniters to see if one works better than the other.

 

Good catch. I was thinking speedo and not tach.

 

I would inspect the trailing ignition system right from the start. The tachometer is fed by the trailing ignition and specifically the ignitor. A failing ignitor, loose wiring or faulty grounds can cause issues such as yours. The J109 ignitors are screwed to the distributor body via two small screws. They can be removed and tested per the FSM. I would first check to make sure the connector on each is seated properly and then check for breaks in the wiring, test the ignitors, and then pull the cluster to check for bad solder joints.