Keep your car idling and disconnect the coil packs 1 by 1. When there's no noticable change with a particular one unplugged, you've possibly found the faulty one.
Now swap it with one of the ones that you previously found out were working ok, then go back and try the above method again. If the same coil pack is missing, you've confirmed it's dodgy, and nothing to do with the wiring.
Make sense?!