1996 buick sklark gas gage werid

I have a 1996 Buick Skylark with a 3.1 engine. The gas gage reads empty when you first start it. It will slowly move up to the proper spot after 5 to 10 mile. If you start and right away o to the back of the car and bounce it about 8 times. The gage will go up and read acuratly. What could this be. Looked for loose wires, did not find any.

Check your float in the gas tank..if thats not it...it might be some electrical unit or ur sending unit..maybe the fuel line assembly..but check the float in the gast tank first

How do you check the float? Do you have to take the tank down?