got a pic of the pipes?
If you are getting a misfire, I would suspect that one of the coil packs harness isnt pushed in far enough or you have damaged the wiring to one of them (the wires get broken due to heat)
to test which one is knackered, start the engine and remove the harness at the back of the coilpack, the engine note should drop slightly, find the one that doesn't drop and that the culprit, if the wiring is ok and its all connected right, try swapping one of the coil packs over to check if its the coil pack or something else.
did you replace the spark plug gasket at the same time? its normally in with the set and stops the oil going down onto the plugs, if you do the above next step is to test the spark plug on that cylinder
