[P.S.]

Thinking outside the box... Has anyone checked the coil ground? The coil needs a ground also, and it's usually through the clamp to the firewall. If the coil or clamp is painted, or if paint isn't removed around the mounting holes in the firewall, the coil would have a poor ground.

Also, if paint is removed around the coil clamp bolt holes, and then begins to rust or corrode, that ground is compromised. It would show up after some vibration. Just tightening the clamp hardware won't fix it, the area needs to be cleaned, the clamp needs to be cleaned, and remounted.