If you have a different dizzy that you could swap out to see if it runs that would save you buying parts until your sure that's the problem. I had a early 70's van with a 318 in it that did the same thing and it really puzzled me for a while, but you have to go through the motions to get to the final conclusions. But it acted just as you described, run one minute and not the next. Although mine was much slower to go out so it left me sittin' more than once. The last time it did it it almost was if I was running out of gas when it finally quit. It would start and try and run but it was a chore to keep it going.

PS remember to use a brass feeler gauge to set the air gap to .008