Car running rough, sputtering.

I installed 6 new spark plugs and boots, replaced 2 coolant lines. Had two rear plugs that were pretty black all others seamed to be burning perfectly. After plug installation car ran great for two days and now it's sputtering and lack of power. Had catalytic converter checked and ok. Is this coil packs? O2 sensor? Sons car, has very little $ to repair and 3 more years to pay on it. 115k miles on it. Code said intermediate misfire. What is my best strategy in determining the lack of power and sputtering! Is there any common problems with this make with that amount of miles on it.