On my 66 6 cyl, I had the same problem you are talking about, I had replaced the coil with a repro yellow top coil, someone on this forum had talked about the coils polarity being reversed and causing this problem. Took it to autozone, found out mine was also reversed. Replaced (non yellow top) cured problem. Autozone or some of the other shops should probably check it for you no charge. Just a thought.
1966 Red Conv (200)
1966 White Pony Int.(289)A Code
1966 Coupe (289) C Code
MCA 36048
1966 Red Conv (200)
1966 White Pony Int.(289)A Code
1966 Coupe (289) C Code
MCA 36048