Mine did that too when I first got the car. I checked and cleaned the connections (including the small nuts that hold the ribbon to the back of the gage) with no improvement. I replaced the gage mechanism and it was sorted out.
I have found that cleaning the grounds at the front of the engine compartment helps with gage stability. My tachometer would read inconsistently. I cleaned all grounds and the power distribution blocks in the engine compartment and now it is rock steady.
84 Spider - Long gone, but started the addiction; 85 GTV6 - Current project, Supercharged, Mega Squirt; 88 Verde - "Milo" It has a name, must be a keeper