i have SPF GT40 Mk2 P2116 Roush 427 MSD ignition. Motorcraft/Autolite plugs. lately engine pops intermittenly. at idle, at speed, accelerating, decellarating, not backfiring. not consistent. pressed down on plug wires at plugs and dist. all tight . car runs fine. changed plugs once. log, tedious process. #3 plug took forever. this just started. any suggestions, hints?