The gauge in my Mk I showing the fuel level in the tank madly vibrates.
This happens even with the car stationary. It appears to be triggered by the small voltage fluctuations when the fuel pump goes through a pumping cycle. But once triggered, the needle in the gauge vibrates for a long time.
Since this is related to the fuel pump, I expect a ground issue somewhere is the trunk.
Fuel pump and fuel gauge do not share wiring.
My MK1 Fuel gauge is steady, but all wiring is new (wire loom from SNGBarratt )