12.0 isn't that rich on a duratec. I tried everywhere from 13.5 down to 11.5. From 12 to 13 there isn't that much of a difference. It could be leaned a touch but you won't go 'Holy shirt this is better' at 12.5.
Whatever the issue it's mechanical. My guess is the coil or a wire is arching. All of your grounds are good right?
