I have a 1998 Mustang GT had it in and out of the shop and they dont know what is wrong with it. It has a check engine light on for P0305 cylinder 5 misfire and that is all. The car has a slight miss when it gets warm in 3rd 4th and 5th when I try and stay a certain speed but only under light throttle5-10%. Half to full throttle and the car is fine. Other than that the car runs great. I've replaced spark plugs, coil packs, wires, fuel filter, and throttle position sensor all with oem. My mechanic checked to make sure it was getting spark and also checked the injectors and swapped them around also checking fuel pressure and made sure all had correct voltage and resistance. Except the plugs kenne bell suggested copper plugs. Car has a 2.1 kenne bell kit on it. Any help would be greatly appreciated. Because I am stumped.