First of all: what's the modelyear and the engine code (positiion 6 tru 8 of the vin).

Is the exhaust emission been checked with a gas analyzer? If yes, what are the values?
Have the fault codes been read? And if so what were they? Preferably with numbers.
Misfires can also be cause by a mixture problem - usually to lean.
That may be caused by defective sensor(s), air leaks, low fuel pressure etc.