Whenever I am driving my car and if the gas gauge goes a little bit under half full the gauge will go all the way down to 1/4 of a tank when I know there is gas in the tank. When the tank is full it is fine, but when it moves under a half it goes down without coming back to its original position. Even when the car is turned off it will move up and while your driving it goes right back down

Thanks for your help.

SD