I would verify that you have a good connection between the spark plug threads and ground when testing for spark. If just laying the spark plug on top of the cylinder head fins there may be enough corrosion on the surface of the fins so that spark would appear to be intermittent and / or weak. In some parts of the world where someone is situated near salt water (England near the coast comes to mind) the corrosion might even become bad enough that the cylinder head may have some resistance between it and the actual engine block resulting in a poor connection to complete the secondary ignition coil circuit. Could check for resistance between the spark plug threads in the head and the block - should be zero. Another possibility might be that the surface of the points contacts are becoming coated with an insulating substance - engine oil seeping into the points area from a leak? A broken wire internally providing an intermittent connection also is another possibility. If this is a magneto system and a switch grounds out the points to shut the engine off the switch could be intermittent and partially shorting the primary ignition circuit to ground in the on position. Just some things that come to mind that you may not have checked - it could also be that one of the new components is defective. Hope this helps.