The issue you are describing is more than likely caused by the sending unit in the fuel tank.
The contact in the sending using acts a resistor, when the sending unit float goes up and down it essentially registers different resistance which translates to the fuel gauge. (see picture below)
With the constant up and down movement, over time the contact wears off and can give you incorrect reading at the gauge.
Please let me know of any questions.
Images (Click to make bigger)
Wednesday, August 11th, 2021 AT 7:40 PM