#3 plug arcing at its base where it screws into the head....

If I had to guess, and I do, I would speculate that when BackYard Bob screwed the spark plug in cockeyed it has been that way since Herbert Hoover was president. The plug gap can now probably be measured with a yardstick. Consequently, the voltage required to jump a spark from the plug wire to the cylinder head is less that than the voltage required to fire a spark across a two-foot spark plug gap under compression pressure. That plug probably has a nice fat carbon track down the side of it, thus, errant sparkage.

I reserve the right to be full of sludge.