As you know I'd want to pin it to fuel or ign first. Sounds like you have a killer fuel delivery system but I'd T at the carb inlet & run a neoprene line to a vac gauge with a psi side & tape it to the windshield & need several psi at the offending rpm/load. since you've been dealing with this for awhile I'd go ahead & go to the trouble of subbing in a regular electronic ign & see what happens. Is rotor phasing close enough? NAPA has an Echlin rotor with a .060" longer blade that doesn't fix phasing but helps it as it reduces the distance the spark has to jump. MO-3000 for $8 dollars & change out the door. As said check preload. I'm gonna WAG that it is the Might Sputter & Die system. Holler how it goes.