I'm definitely not a professional mechanic, but i've been amongst the oil and the spanners long enough to have the view that it can be pretty much impossible to pinpoint the cause of something like what you're describing on paper.
You have to be there and experience it first hand, and even then, it's not a given that an accurate cause can be diagnosed without trial and error via a test ride or two.
How many things could it be, right?
Engine side: Engine running lean at a certain throttle opening came to mind. A tank of slightly stale fuel perhaps? Dodgy spark plug maybe?
Heck, even just something slightly loose close to the handlebars could give that sensation, but then again, if you're in gear and moving, it could be whole stack of other causes.
Gearbox/drivetrain : Possibly, but I think unlikely due to the frequency you're describing. Would tend to be a lower frequency and it would probably get more pronounced as you speeded up.
Other potential causes: Sometimes tyres can give a different vibe at certain speeds etc....
What's your gut tell you?