The trouble code is P0303 on my 2003 LS V8. When engine on and gear in parking or neutral position engine working isn't optimal, it seems it hasn't enough rpms and the car is shaking a little. Also the engine has lost a power. When cleaning the code the engine begins to work fine, but after two or three day driving the check engine light is on again. I've changed the ignition coil of the 3rd cylinder and the spark plugs too, but the same happens.