Gauge registers 3/4 full when full, then needle slowly goes down to empty. Flucuates between empty and 3/4 full while driving.

You could use two resistors, Two different ohms specs for full and empty. Place resistor between proper wire colors in diagram. Turn on the key, If gage reacts properly to full and empty resistors, then I think the problem is the sending unit in gas tank. If not, the problem could be almost anywhere, wiring circuit--pcm--instrument cluster. I did see trouble codes relating to fuel gage sending unit. I hesitate to get into that? good luck

Bad sending unit, bad ground, bad gauge, I have seen sending unit go bad they will work part way.

need to replace the sensor

The sender unit in the tank is the most likely cause. To test the panel unit, remove the wire from the fuel tank unit and gound it. The panel gauge needle should move up full scale to hot. Remove the wire from the ground and the gauge should return to empty. If this test is ok, the tank unit needs to be replaced.

the float is punctured

It sounds like the fuel sender is bad in the fuel tank.

the only thing that will cause this symptom is the fuel tank sending unit.

probly a bad sending unit

It is a common problem. It is a faulty in tank sender. You can just replace the sending unit by removing tank.

make sure the antifreeze is full. if so, replace thermostat and refill