Hmm, dude i had the missfire problem too when i changed from copper to iridium plugs too. (cost me 80 or so dollars too fk that), but dude as alot have mentioned, regap the plugs to 0.8 with a regap kit, easy as to do, i did it myself.
Theres also a thread for this as well where some have tapped up their coils which i have also tried whilst the plugs were out to regap. My car hasn't miss-fired at all as of yet.
Hope this helps.
m.