I just bought a used XLT and I've noticed the gas gauge is acting strangely. If I fill up the tank, the gauge doesn't swing up all the way to FULL until I've driven the car for about 100 miles. It slowly moves up to full as I drive it. Then it slowly moves back down as the gas is consumed, as it should. Letting the car sit doesn't allow the needle to go to full--it is only through driving the car. What can be done about this???