[Sunnybrook Farm]

Could be the manifold gasket leaking or some other vacuum type of leak. Of course it could be dirt in the fuel, what kind of filter do you use? Sounds like it is something fuel related since your distributor is rebuilt. It will be a process of elimination, I have sprayed something like carb cleaner on the gasket area of the manifold while running to see if it shows a leak is sucking air into the motor. If you have a spare carburetor, swapping that would tell if that is the problem.