Hi Adrian, the most likely causes are:
1) the float in the tank. If the plastic container (float) is full of petrol it fluctuates widely.
2) the spring tension on the tank unit resistor is not sufficient throughout the span, causing open circuits. Bend it in.
3) the earthing on the fuel tank (and or) the earthing on the back of the dash. Also the reliability of the battery to frame.
4) a faulty gauge. Both the tank unit and the instrument can easily be tested with a multimeter.
The voltage regulator will have virtually no effect.
The resistor (light globe) goes on the back of the instrument and will only help by dampening the needle a bit if all else is well . Trial and error to the globe wattage is required.