Changed the plugs and they were pretty bad. Looked like the originals. Very "sooty", points nearly gone. One was even missing a large part of the ceramic but I could tell that was old damage. But no presence of oil. It is running a little smoother but still missing. Can a clogged injector cause this? Maybe bad wiring to a coil? Would a code for the oxygen sensor before the cat. cause this? Amazingly, the cat. seems fine. No smothering or apparent back pressure, very free flow of exhaust. I am guessing it can be many more things than the above. Ideas where to start or stop gladly considered!