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First check to see if the sender float is being interefered with by something. It could be the fuel pump strainer, it could be the plastic container in the bottom of the tank which keeps fuel from sloshing away from the pump.
If that isnt it, check the sender itself. Mine had a purple wire leading from it to the top of the wiring assembly. One can pry the clips off it and check inside. There is a coil of thin and delicate wires, tha a contact slides along. check to see if the contact is clean, you might clean it with some sandpaper. be careful with the wire coil. There is a tech tip for this here on the forum, i believe.