One possible reason for that peculiar behavior might be a malfunctioning shock absorber – it may be expanding under load and binding up the gearbox. But I’d try to exhaust the powerhead options before tearing it off again. Recheck that the points are properly set, maybe try a spark tester to see if a coil misbehaves, synchronize the carb and magneto, and I’m sure wiser heads will think of many more options..