Check the distributor shaft for side to side play. The shaft runs against the cast iron housing without bushings, and there's supposed to be a spring and a phenolic thrust button behind the grease cup in the housing to keep the shaft pushed against the side of the hole it runs in. That spring can wear out or the grease can get solid, allowing the shaft to wobble and change the point gap. That alters the timing between cylinders and causes the idle speed to "hunt" instead of being steady. Richening up the mixture by restricting the intake and getting it to smooth out sounds like you're dealing with a small vacuum leak. Try moving an unlit Propane torch around the carburetor base and the intake manifold. If the idle evens out when the gas finds the leaky spot, fix it.
Jerry
