Pontaic Grand Am Missfire

This is the first time I have worked on a quad four engine. After complaints from its owner {mother-in-law} about
sluggish performance, I drove the car for look see. she was not kidding . It felt like it was running on two cylinders.
I removed the coil tower cover and found telltale signs of carbon tracks from arching between the plug boots.
I replaced the coil tower,plug boots and plugs. I fired it back up and it ran like new. Now it seems to have an annoying slight but very noticeable miss when idling. The owner informs me that it was not there before I worked on it. I was hoping someone might have a suggestion as to what I might look at next. Thank you