As Bruce says the condensor is a possibility. They either work or they don"t and you"ll see some burning or pitting of the points if the condensor is at failed. Another possibility is a bad key switch. If you find a weak spark, put a jumper around the key switch and see if that clears the problem.
Check the spark and report back. If the spark is OK then a restriction in fuel delivery could be the problem. There"s a screen in the fuel inlet elbow of the carb that can partially clog and give similar symptms as acan a clogged fuel vent in the fel cap or a restricted tank outlet.
Check the spark and if that"s fine then look to the fuel.