long time ago, I was having the same problem in a car with points, dist, coil, battery:
I tried it all and Finally I went to electronics store and bought 3 of those itty bitty milliamp bulbs with pigtails that light up and draw almost no amps. I made very long pigtails. I taped all of the bulbs on the dash of the car. I hooked one to the hotwires between at the switch. I hooked one at the coil. I hooked one to the dist. on the low voltage side . all burned red when running. one day, the light at the distributor went black/out just as my engine died; other lights continued to burn. I knew it was in the distributor. when I disassembled the dist, under the point plate I found the wire was broken in two pieces but sometime they touched and sometimes not. I replaced the two-inch wire and it ran dependably.
this was my technique to find the problem.
fordman