When I am driving, the car runs on 3 cylinders when i hit the gas harder. You can hear it work on for, then when you punch it, it gets a nasty growl and runs on threee and goes no where fast. I replaced the spark plugs, wires, cap, and rotor. When I changed all this it worked fine for about a month, then I took a plug wire offf to take the valve cover off to check somethin, and it is doing it again now. What is causing this?