sounds as though a ground is not hooked up. you just did a verter install right. i would seem to think it was a wire not hooked up or something simple. if it is a random missfire then it seems to be all cylinders at one time or another. if it were a bad plug it would have nailed down that cylinder.i suppose it could be the verter but that seems far fetched. man other than that i dont know. there are so many places to start. and i dont think it will stall up to what it is suppose to if it is missfiring. either that or the verter is having issues.