....How about take one new, out of the box, plug....and go around the engine replacing each plug until you discover (perhaps) a faulty one. Use anti-seize paste on the threads and be sure not to exceed 100 INCH-LBS. (Less than 10 ft-lbs.) I have seen so many plugs and cylinder heads damaged by too much torque. It doesn't take very much torque to stretch a plug's base, separating the metal from the insulating porcelain ... creating a high voltage leak.