BB, because you think it is sensitive to HT lead positions, it sounds as if the next step is to measure the resistance from the HT terminal (spark plug connection) to both ends of the primary winding. It should be around a couple of thousand Ohms, and it shouldn't change at all when you wave the HT lead around. If it changes from 2k Ohms to much more than that, the HT lead is a goner.
If the HT lead is OK, there are two plausible explanations left. The first and more likely one is that the capacitor (condenser) is breaking down. The second plausible problem is secondary insulation breakdown, which is relatively difficult to test for, though repairers often apply some kind of test to detect it. I haven't actually seen the test.
An old ignition scope would find the explanation in a flash if you spun the crankshaft with a drill, but I think they fell out of favour several decades ago. A pity - one of them baled me out of a mess in deepest Louisiana once when my Camaro developed the stumbles whilst on a 2 week road trip. Less than 60 seconds to find that the highly inaccessible points in the 327's window distributor were all burned up, and he replaced them in about 3 minutes more. Those were the days. $12.50 all up, including the new points.
