I am pretty sure that GeezaGillard had a similar problem to this.
In his case it wasn't so much a bucking/jackhammer as just a bogging over-rich feel.
But I'm pretty sure it was happening under the same conditions.
Basically he would be in top gear doing about 80mph and would ease off the throttle down to about 60 mph and then on re-applying the throttle it seemed to bog down when throttle was applied again, but only for about 4 seconds and then it would clear and be fine if you kept the throttle on. The bike would always accelerate fine except for just after coming down from higher revs.
Jim, if you keep the throttle on and "power through" the bucking, does it clear up in a few seconds?
In GeezaGillard's case he was able to track it down to the carby needle valves not sealing very well in their seats.
He used a bit of brasso on the end of a cotton bud to clean up the brass seats and put in new rubbers on the tips of the needle valves and the problem was gone. Magic!
I guess the float bowls were just momentarily slightly overfilling somehow when coming down from higher revs/throttle, causing bad/rich carburation?
Seemingly due to the needle valves not quite sealing off soon enough?
Perhaps in your case the momentary slight over rich condition is just enough to be on the verge of stopping the fuel igniting?
This would make the firing intermittent causing a bucking effect?
Just thinking out loud. I have no idea if this is your problem, but worth investigating?
It is possible I guess that GeezaGillard actually unknowingly did something else during the needle valve refurb that was actually the fix, but I seriously doubt it.
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