It's always nice to do one at a time. So you know what was the cause.... Take this from a technician, as being a parts changer is not a diagnosis. I've seen guys throw hundred of bucks at once on an engine when it was only a 3 dollar gasket.
Cap and rotor is always a great start. The low idle needs a hot spark, without it, you get a surge in that baby. I change mine yearly like matt said.
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