I can remember back in 1971, I believe, having this same problem with a 69 R/T, 440 six pack. Ran great for two years, then I cleaned the engine on one bright sunny day. All of a sudden it ran very rough, or rougher than usual. Then it stopped. Can you believe that I had it into a Dodge dealer in the Quad Cities, couldn't find the problem, then to a local garage who thought that he had it fixed, just to find out not. Several starters, battery, plug wires, coil, the only thing that I missed was that ballast resister. Finally sold it, AS IS to a guy returning from Viet Nam. That was the last that I saw of it until about six years later when I came up behind it on the street. Could always recognize that R/T with its Dana so visible from the rear. Ballast resistor was what he changed, first thing, and it ran like a top. Expensive stupid lesson.