[HO455]

Try putting your timing light on each plug wire and running the the engine up to the rpm with misfire and see if the light flashes indicate a misfire. Does the flashing show smooth movement as the timing advances and retards on each cylinder?. This should help you decide if the problem is fuel or ignition. My first thought was that you have a bad ground inside the distributor at low RMP (advance) and once the advance mechanism moves the plate enough the ground is restored and the engine runs as it should. Have you tried disconnecting the vacuum line to the vacuum advance and running it?
I would concentrate on the ignition side before diving into the carburetion side. It would be easier to swap the distributor with a simple point style to prove the ignition is good or bad than swapping carbs. The fact that the problem gets better with RPM leads me to believe that you don't have a cap or plug wire problem.