Did you ever figure out the cause? I have a 2007 dodge nitro with 3.7l . I've had a code pop up with cylinder 6 misfire secondary ionization. I've changed injectors, coils, and spark plugs. Even cut off my cats and still at 45-50 mph the engine misfires. I want to add that I've replaced the radiator and water pump already as well. I'm to a scratching my head point to fix this misfire. Hopefully whatever u did can help me?