Years ago, I had a similar problem with my 1954 Buick. The problem turned out to be in the ignition system. A service station that had a scope found that the firing voltage was too low for the spark plugs to fire reliably. Scopes were not a common item back in garages back in those days. At any rate, an ignition analyzer might either pinpoint your problem, or eliminate the ignition system as the cause. Your symptoms sound similar to what I had with the Buick.