[tdangle]

I think you need to focus on the HEI dist. I had the same type of problem years ago and it ended up being the center button of the HEI cap not springing down and making contact with the rotor. Not sure how it ran at all. I would look in that direction. Also for carbon tracks in the cap. If the electronics of the HEI were bad, I'm pretty sure it just wouldn't work at all not be intermittent. Follow the spark path from the coil (in the cap) to the rotor and to each stud (going to each plug)

Good luck and let us all know