Start by checking the coil towers. They have been known to rust, and it usually is just one to start with. If those are clean, check the resistances of the wires to see if one is a lot different than the others. The longer the wire, the higher the resistance will be. Also check the condition of the coating, that it hasn't worn through anywhere.
Looks like a bad plug wire. Took the boot off the coil and it was black and grimey under there. Been working at our air show here all weekend so I'm going to mess with it Monday morning. Hopefully, I can get my arm back behind the engine to pull the plug wire off.