Honda C100 still has issues :-(

I’m running out of ideas about what else to do with this.

I have a issue with the bike that has got be really stumped. It starts and runs beautifully but at high RPM, high load (95Kg of me and a hill) it misfires like it has an ignition fault, on/off/on and eventually stops. It gets worse the hotter the engine gets. After a few minutes it will reluctantly starts and goes on a bit longer until it does it again. It does it after just maybe 30 seconds of starting it and going up the hill from my house. Very repeatable.
I thought it was probably points and condenser, so I replaced them. No difference. I thought it may be some dirt in the brand new carb I fitted, so I cleaned that. No difference. I then got fed up with points and condenser (always disliked them) and splashed out $250 on a CDI conversion, magneto and everything. No difference. I then thought it might be the coil, so I fitted a new one. No difference. I thought the ignition switch might be faulty, so I bypassed it, no difference. I have stripped the engine completely (as part of the restoration to paint it) and the valves have been lapped, checked with engineers blue and clearances set correctly. I’m now wondering about the “selenium rectifier” and whether that would give me those symptoms? The bike has a brand new battery and the fuse did blow at one point, not sure why though?
I don’t think it’s a fuel related issue, I get good flow from the tank and it feels too sudden for fuel, like the spark is breaking down under the high pressure combustion of high load, high RPM.
I’m hoping to buy another wreck of a bike so I can swap some parts over for a better diagnosis.
Any words of wisdom would be much appreciated.