how old is your wiring harness?worth looking into. a bad harness can contribute to a misfire. a mechanic in general at a shop/dealership isnt necessarily gonna try to fix your problem the first time around. they usually try to fix as much as they can to get the most money if that makes any sense. anyways ive owned two z's and the wiring harness's on both were pretty $h!ty. after i replaced the harness on my current z it ran much better (on 6 cylinders instead of only 4 lol).you can replace coil packs injectors and a long list of other parts but if you arent getting the correct connection/current through your wiring then your car isnt gonna run right.