Does sound like a failing system voltage regulator.
The thermal gauges, temp, gas, oil, always were designed for 6 volts. So back when all the vehicles changed from 6 volt to 12 volt systems, most every mfgr simply added a mechanical voltage limiter for the same gauges ( GM is one exception ). Most cases this limiter is a small metal can thingy attached to the back of the inst' panel, Chrysler, Ford, VW, etc...
There are exceptions, including models where this limiter is housed inside a gauge. Early Imperials and others had it in their temp gauge. Other models have it inside the gas gauge. So....
Any gauge should require 2 electrical contacts, current in and out. If you find a gauge that has more than 2 contacts, it has something more in it. If your A-body fuel gauge has 3 contact posts, one of those is 12 volts going into the limiter. That 12 volts is chassis grounded via a small piece of metal on the back of the gauge,,, and like most everything else about the car, The instrument panel must be attached/grounded to the dash for it to work ( completes the ground path). Quite a few wires to a inst' panel and not one of them is a chassis ground wire. It's a good idea to add one, Comes in handy if/when you want to test bulbs, limiter, and such while the panel dangles from its harness connectors.