Unfortunately the problem may lie with your EZL transistorised ignition unit. Item 8 on your diagram. These are bolted to the inner wing which acts as a heat sink for the electronics. Unfortunately with age the heat sink compound between the wing and unit dries out-it overheats and then you begin to get problems as the unit warms up. Lots of these are failing now and they are expensive to replace. They can be repaired if its just the output stage but if the ignition curve control chip is toast replacement is the only answer- and they are not cheap. However- one stage at a time as Dieselman suggests -test the spark. The best way to trouble shoot these old systems is to find a garage+ knowledgable mechanic+ old fashioned oscilloscope engine analyser. once hooked up you should be able to display the ignition HT firing lines for all cylinders- this should tell where the problem lies i.e. which cylinder is misfiring or whether the coil is giving the correct output etc.