Quote:

When the plug is grounded there is an intermittent weak orange spark between the insulator and the plug body



This would make me think bad plug wires.
The rest is kinda mystifying though.
Could be the wires are shorting, and when you move them, it stops the short.

So replace the plug wires, they're cheap enough, and see what happens.

Contra todo mal, mezcal; contra todo bien, también