You might have a sending unit with a bad wiper. The part that rubs on the wound wire that is the resister. Try to increase the tension on the wiper and see if it is more stable out of the tank. Hook up every thing and move the float up and slowly watch the gauge and see what happens. In my truck all you need to do is pull the screws that hold the seat back and remove it to gain access. then be careful because you do have an open fuel container right there where you are working. But you can pull the sender and work on it from the seat. Use a jumper to ground it and try it.

Steve H