My car is hesitating during acceleration, chugging during deceleration, and generally rough riding as if a small amount of gas is going on and off. It happens 95% of the time and has become dangerous when getting on the highway. We have changed the spark plugs, distributor cap, wires and fuel filter. Someone has suggested the regulator may need replacing but I don't know where it is located, whether it is in the gas tank or the engine. Thanks.