INTERMITTENT IGNITION ISSUE

EDIT: as to the missfire;

My guess is that one of the plugs has a cracked insulator.
Because this is sometimes very hard to see, I would change all the plugs. but I would also try to figure out how/why this happened, which is usually detonation or pre-ignition.
If I'm right, you gotta fix this. If the plug insulator breaks up and falls into the cylinder, there is no guarantee that it will go out the exhaust without doing damage.