Check the inside of the tailpipes. Are they grey or black? If grey, the fuel mixture is ok. If black, the mixture is too rich: The power valve is most likely leaking & needs replaced. Is the fuel filter plugged? Could there be a pinhole in the fuel line from the tank to the pump or is the pump itself weak. Any of these will cause it to lean out under acceration, there-by starving for fuel. Next check the distributor. Is the point gap correct? Do the points & condensor need replaced?
Is the distributor shaft wobbly or tight? Look closely at the distributor cam lobes. Each lobe should come to a point & they should all be even. Are they worn?
Try these then let me know.
Bill