Probably a bad plug wire, are they the originals? If so, they are too old and tired(Increased resistance). The spark looks for the easiest way to go, it found a way to go that it should not take, but even if you clean off the carbon, it will try to take the short cut. I would get new plug wires and use the spark plug goo, I would not use vaseline. Don't forget the anti-sieze on the threads. I got aftermarket wires, but the OEM ones are at least as good.