I would suggest using external coil with ballast if you have points.
I still can not see a motor operating well under load and bad when load is relieved. It seems like gearing/clutch/governor. I was thinking it was a 4 speed? You have still not stated the speeds you are obtaining in each gear. Whatever your final gear, 3rd or 4th (without overdrive)the engine/tranny ratio is 1:1 to the pinion. I know you are not doing this but is sounds like when someone shifts from 1st to 3rd or 2nd to 4th. Some others thoughts: valve float, points break down, shift linkage mixed up/tranny sliders or it's normal..... Let us know what you figure out.

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