Last weekend the car was running really rough and vibrating badly. Through process of elimination we found a bad Ignition Coil. We replaced it and that seemed to fix the problem...this morning the same thing started happening again...where should we start looking now??

 

So I tried changing the Oxygen sensor, which obviously needed to be replaced after we took it off and looked at it. But the car keeps doing the same thing. It will run fine for a few days, maybe a week...then all of the sudden it starts sputtering and shaking. We always check the coils and sure enough one of them isn't firing, always the same one. Where should I go from here?? Obviously putting the Oxygen sensor in didn't stop the problem. Please help!!