Some times just an earth problem can do it, eg some one has just fitted gas to the car, the gas gauge can interfere. The cluster pack might need to have the plug connection cleaned. One thing is for sure i would do , is try the sender, some times people putting a garden hose down the neck of the tank to get fuel will damage it or it . So try the sender or get an auto elect to suace it for you as it does not take much if you are just poking around to blow instruments and guauges up. They are simple machines of that era and wont cost much to find the problem from a professional, providing you dont have a bad problem.