My wife drives a 2002 PT Cruiser with 110k miles. Check engine light is on and it has stored code P0303, miss-fire on #3. We bought it in 2003 with 6k miles on it. The thing has always had a rough idle, and often jerks when starting off cold. After several overnights at the dealer and never fixed, we just put up with it. Now that the check engine light is on, the rough idle is much worse. Plugs were replaced around 80k. I checked #3 plug and it looked normal. Same for the wires. As a test, I switched the #3 plug with #4, and switched the #3 wire with #2. After clearing the codes, same P0303 code appeared almost instantly. What should I look for?