Hi;
I would have tried new plugs and cables, and I'd consider throwing in a new rotor and cap as well. Your plug looks similar to one on NGK's website:



Accompanying text:

"Bridging is when deposits accumulate between the center electrode and the outer shell or base of the ground electrode. These deposits can form a "bridge" for an improper spark path from the center electrode to the outer shell, this will result in misfiring. Depending on application, there are a variety of self cleaning designs used by each manufacturer to reduce bridging."

Another view on bridging:

"Firing voltages should not vary by more than 3 kV cylinder to cylinder. A cylinder that shows an abnormally low firing voltage probably has a grounded spark plug (deposits bridging the electrode gap), or a shorted ignition cable. A cylinder that shows an abnormally high firing voltage compared to the others likely has an open ignition cable or a plug with a wide gap."

Erling.
--
My 240 Page