Could very well be an improper end connector. I don't know how yours work. "Some" require stripping insulation to the core, folding it back into the connector.
If the trouble is one cylinder, plug, or wire, you can try a grounding probe. Pull the distributor boots loose. "Rig" a grounding probe. you can use a 12V test lamp or a small screwdriver and alligator lead. Ground each tower one at a time and see if the arcing goes away. "that cylinder" is your trouble.