I just recently purchased a 1998 Jeep Grand Cherokee Laredo straight six 4.0 liter. It seemed absolutely flawless! However, just today, every time I turn on the car, the gas light comes on and the gauge says I have no fuel! I actually have 3/4 of a tank though. But once I put it in gear (it's an automatic), and start moving the gauge jumps back up to where it should be. What causes this? And should I be worried about it? Thank you!