Well you have replaced the electrical side of what could have been the problem.
You first need to find which cylinder is mis firing so the best way to do this is just in case the car wont run and stalls have someone sit in the car. Start the engine and slightly rev just over 1000 rpm take an insulated pair of pliers and carefully apply the pliers to the cap end which joins onto the spark plug pull the cap away from the plug do not pull on the lead and make sure you are not touching the engine or anything metal around the vehicle just in case you get a spark.
Carry this out on all cylinders. Listen to the engine if the engine dies down or tries to start then this cylinder maybe ok. If you pull any lead away and nothing happens the engine does not try to cut out then suspect that cylinder.
You could have either a blown head gasket? any oily residue in the expansion tank the tank in which you add water for the engine? is the engine overheating? these are some of the things that you can get with a blown head gasket.
If not the gasket then it could be an exhaust Valve not seating properly could be burned or chipped.
Compression ring on the piston.
There are other hints as to what the trouble could be but this i would have to go into greater detail.
I would hazard a guess you have a blown head gasket?
Carlito
October 2007