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Senders are a float on an pivoting arm that moves a contact (bronze?) across a resistance to make a rheostat. The constant movement of the car causes the fuel to slosh, wearing out the contact or the resistance element.
Looks like it is a common problem. Easy replacement, but sometimes requires removal of the fuel tank from the car.
I had an old truck that required a fuel sender replacement. It was quicker to take a hole saw and make an access hole in the floor than it was to remove and re-install the entire fuel tank... If you remove the tank, replace all of the rubber hoses at the same time.