the gauge is powered by the 5v Limiter.
The limiter is actually fed 12V and the points inside the limiter flutteropen and closed on about a 40% duty cycle thereby limiting the output voltage to 5V.

that 5V is fed to the gauges via the circuit panel.

the sending unit signal is a variable resistor to ground.
That variable resistance is what makes the gauge read.

You likely have an intermittent ground issue with the sending unit that is making you chase your tail. BTDT you can try a 10, 23? and 70 ohm resistor between the sending unit connector and ground MAKE SURE YOUR GROUND IS CLEAN and that your jumpers are not intermittent ( been bit by that more than once).

The gauge should read High, Mid range and Low with each resistor. Give the gauge time to react when connected as they are slow to respond.