Are you sure youre not just a little less than 1/2 tank when that happens? If its a fuel injection car, you have to make sure the gas level in the tank is above the float or it will not run properly...sometimes even buck or stall. Youll notice also that going up or down even small grades will affect the way the car runs as well. If you notice the car runs better with a full tank, then have someone look at the float for you. Or drain the fuel tank completely (sometimes dirt or sediment on the bottom of the tank, and water can cause that to happen). Good luck