My gas gauge has been going up and down the past few days, it doesn't stay in one place. It was never like this before, but I put some Gumout fuel system cleaner in there and topped off the tank. Next thing I know my gauge goes up and down constantly. WTF?? Anybody else have this problem?

 

There is a float that sits in the tank. It is called the sender and as the float moves resistance is measured - you need to replace your sender but that means dropping the tank.