Time for an old stylee diagnostics. Check (or replace) plugs, leads, dizzy cap, rotor arm, points, condensor, coil. Make sure coil LT circuit is sound. Check fuel tank for water ingress, check in tank filter isn't clogged, check sedimenter bowl and filter are clear, check lift pump is working efficiently, replace in line fuel filter (if fitted) check carb for water/crud especially the slow running jet. Check carb/manifold for air leaks. Check air filter isn't crudded up, make sure air filter hose isn't collapsing under suction. Take a college course in vehicle maintenance or buy a new car that tells you what's wrong all by itself.