If your fuel level gauge stays still when the car is still, the problem is due to sloshing fuel in the tank. When the vehicle is in motion, fuel is allowed to move back and forth and move, or “slosh” around. Many late model tanks have internal baffling and some fuel cells use a foam insert that help keep fuel from moving around too much. When the fuel is moving around, it is moving the float up and down on the sender, which causes the gauge to respond by moving back and forth. Some OEM’s have also used what is called “anti-slosh modules” to battle this problem in production vehicles. Unfortunately, we know of no after market anti-slosh module to operate with an after market fuel level gauge. Some amount of slosh or movement is normal with a non-baffled or a non-foam filled tank.