Could be fuel starvation under load. You may need to pull down the carbs and check for fuel gummuing. This happens if the bike is left standing and the fuel goes stale, evaporates and leaves a residue in the emulsion tubes and jets. The bike runs ok on the primary jet until it needs more fuel from the main jet. Does it smell rich when running? - caused by large fuel drops not airated by the emulsion tube above the main jet. Could also be a coil with a high resistance - compare the coils with an ohm meter - plug lead to earth reistance (make sure you have good contacts for accurate readings).
Has it been wet? - sometimes water will get iinto the jets and sticks in there. This will evaporte and go away but return later when the next water droplet get stuck in there. Drain the fuel (including reserve) and replace with fresh fuel, add carb cleaner (a 1/4 cup of kero/ tank works).
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