Engine miss

Friend has a 99 Cirrus, and the engine keeps missing. We have replaced the EGR valve, and then the coil pack as the computer showed a miss on #3. The plugs were replaced and #3 wire tested good. The battery was relaced about 8 months ago. Is there anything else I should be focusing on here? it is a 2.2l.. It runs good as long as you keep the RPM's up. He went on a 600 mile trip last weekend (gutsy, I must say!) and it ran great almost all the way there. Once he got into city type driving, the miss came back. He could put his foot through the floor and it would barely accelerate. Ont he way back, he discovered if he felt it start to miss, he could stand on the throttle, get it wound up good, and it would be fine for a while. I really do not know where to go next on this. I need a little guidance on what to look at next.

J