Minor plug wire arc

It might NOT be the wire you are seeing arc. Can be crossfire, etc

Check all plug wires for continuity. Pull them off one at a time, hook up the meter, and shake them around, looking for change on the meter. This, actually, is best done with either an old analog meter, or at least a modern digi meter with a bar graph to simulate analog

Examine plugs for cracks. If there's any doubt, replace the plug.

If you can't find anything, examine the offending wire for cracks and damage.

The jumping timing could be an indication of an open, IE open wire or cracked / bad plug. Don't discount that you might have lost a ground electrode on a plug!!!

And of course make certain the firing order is correct