Have you looked at the vent line under the bike for obstruction? Or try pulling the tip over valve out and take it for a ride as that could be sticking. If those are OK then check float level , doubt that's the problem but if it were low it would give you trouble under higher demand. Then I would start suspecting something in the ignition circuit such as the pick up coil or pick up coil gap, the CDI , or even the fly wheel moving on the crank messing with the advance also a long shot but possible. Weak connections at any of the electrical plugs in the ignition. I would go with the easy stuff others have suggested first of course. I would suspect a vacuum leak first though since it seems to trouble you under a demand either that or an advance or tank venting problem ,same reason.
