fiat punto s 98 misfire on idle,

I have a 98 fiat punto s with an idle/low revs problem,,, drives ok and produces no smoke but on tick over runs "lumpy" and with low revs gives an unbalenced vibration, i serviced the engine, ie new filters,oil, and plugs,, on inspection of old plugs i found second plug in from n/s of car was black with dry carbon deposite,, all other plugs were fine,, still i replaced all with new,, still problem persisted on same cylinder,, i then changed the coil packs over with each other to see if moved problem to another cylinder,, to no avail,so im guessing that rules out coil pack/s? also changed ht leads,, still no good,, any ideas would be greatly appreciated !!