I'm clutching at straws a bit here but it happened to my bike so worth eliminating it anyway.
A few years back i had a carb problem and when the mechanic put them back on the bike the second carb rubber had folded in on itself causing a small air leak, so whenever i tried overtaking or just using more throttle the bike would just bog down.
Took the bike back a few times but they couldn't find the problem
so i ended up taking it to PDQ and he came into the waiting room with my carbs in his hand showing me what the issue was.
Because it was the second one in there was no way you could see it whilst fitted on the bike.
Worth a check, you never know!