Not sure about the tab in the distributor? Are you saying the rotor was broken?
I'm a**uming that isn't related to the fuel gauge. The fuel gauge sender in the tank is mounted to the fuel pump "assembly". You have a 91, so you should be able to pull the pump without dropping the tank if the problem is with the sender.
In the Renix years the sender resistance values were Empty = 1 ohm; 1/2 = 44 ohms; Full = 88 ohms. The values were reversed in the later models, but yours is probably the same as the Renix.
Your problem could be with the gauge, the wiring, the sender. The sender is a variable resistor winding that the float arm moves along with the level of the fuel. The windings or contact point of the arm become worn and contact can become sloppy leading to a bouncing gauge when the fuel moves.
Try observing the gauge while a friend or two jostles the rear of the vehicle and see it that contributes to the gauge's erratic motion.