my monte carlo 1997 is running rough. it studders when i take off.when i go to pass someone it also spudders. the codes i pulled from the computer state that there is a misfire in cylinder 4, the oxygen to gas ratio is off, i've change the spark plugs and wires, pcv valve, fuel filter, o2 sensor, and still spudders, but it doesn't do it all the time, please help