My 1975 was doing exactly what you described, and although the fuel pump was pumping, guess what? I took it apart and found very stiff diaphragms. One corner of the pump diaphragm was pulled inwards due to it shrinking and enlarged one of the four corner holes enough to cause an internal leak, and this was enough to make the engine act up intermittantly. Anyway, I got a pump rebuild kit from Sierra, but new pumps have come down in price from $80.00 to $40.00 dollars so you may just as well get a new one. You must always use the clamps on the carb, and fuel line fittings, or the pump will have problems. I suggest you do as I did and replace all the fuel lines, they get quite stiff ans don't seal well, even with a clamp. Make sure to replace the o rings in the fuel line connectors coming from the squeeze bulb, and connector to the tank, if they leak you get air in the pump and it cavitates.
