fuel tank ohms changing

[awesometool]

The fuel Gauge changes when going from accessory to the engine running. I tested the ohms directly from the tank full and empty and it was 0-91 ohms. perfect. I drained the tank and the gauge also went from full to empty following the correct ohms that the tank was showing. The fuel gauge is autometer set to 4-86 ohms and the tank has a 0-90 ohms sender.

I retested with an almost empty tank with the engine off it was 6.25 ohms and the gauge was one line over empty perfect. After running the engine one or two minutes the gauge goes to full and then to 1/4 and stays at 1/4. I restarted engine with no gauge attached and the tank ohms bounce around and settle at 22.5 ohms which would show 1/4 on gauge.
So it definately a resistor problem when the engine is running. I tried an extra ground strap to the tank but that did not help.

What would cause the resistance to change when started.
Thank for any help