ace.cafe...
I woke up last night and was thinking about one thing or another about my bullet, and I remembered your electric problem.
I have a theory....
Your plastic cap is not creating an electrical problem. It is creating a mechanical problem which is creating an electrical problem.
If, like my distributor cap, it is both fairly old and poorly made, there is a fair amount of flex in it.
I'm thinking that there is enough flex in it that it occasionally makes contact with the assembly inside and beneath, rotating under it.
Test it by replacing it, getting it running, and then pushing it with your thumb. If it has enough flex to touch the assembly beneath, you should be able to get your engine to "stumble" without difficulty.
You say you have the Boyer ignition. Not sure what that is or what it consists of, but I would suspect that something is being jiggled just enough to occasionally cause a short, losing your spark on a reolution every so often.
I'm thinking is that - if you system has condenser/points -
a) the condensor is angled out enough to occasionally and irregularly touch the cap, and when this happens it moves it enough to alter the points gap (could be loose too)... or
b) - my favorite.. that it touches the condensor or some other part of the rotating assembly and that the very small insulating washer that the condensor bolts onto is cracked or damaged, and when jiggled, it creates a momentary short. This ha'oenny washer only needs to be slightly out of whack and the engine will begin shorting routinely... I discovered this when I replaced the condensor/points assembly WITH
OUT the washer, and it would work, sort of, but I could see sparking as the distributor circled around and shorted out routinely.
If your system doesn't have a condensor or points, I'm still thinking that whatever you have is getting "bumped" occasionally by the flexing of the plastic cap, and causing a brief short.
My 2 cent theory. And worth every penny of it.