I once had issues a little like you have. Could run well for a few miles, then start to spit and backfire and lose power. Next day, while testing with another component, it would run fine, and then the next day with nothing changed, the problem would reoccur. For me it was something simple, the vacuum advance could get stuck. When I disconnected and plugged the vacuum hose, the problem was gone. A new vacuum advance (adjustable) solved it.
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