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Most likely issue is the fuel level sensor, also called the sending unit. It is located in the fuel tank, attached to the fuel pump. I am unaware of being able to easily replace the fuel level sensor without replacing the pump as well.
My gas gauge did that for a bit, I waited it out and it stopped. I don't know what the inside of the tank looks like, but in my old jeep I had back in high school did the same until one day when I was out wheeling I noticed it started working again. Figured I just knocked it loose to whatever it was stuck on.