dash gauge question

You may find the instrument voltage regulator is inside the fuel gauge. If so, Not 100% of it is inside the gauge. There's a thin piece of metal on the back of that 3 post gauge that provides ground path for the limiter. Because the gauge mounting studs are swedged into a fiber board that warps with age, and since this piece of metal is on top of the fiber board, all connections can get weak. Some owners have had gauges go on and off with bumps in the road. So in so cases just cleaning and tightening connections cures it.
Of course you should check system supply first.