For what it's worth, I had the same problem with my 79 D-100 years ago. It was in fact the dist. pick-up. I did a resistance test with an ohm meter at the dist pig tail that confirmed it was dead. The readings when checking at the pig tail should be between 150 - 900 ohm or another way is connect one meter lead to the pigtail and ground the other, if the meter shows any reading the pickup is bad.