Shot in the dark, but the clutch maybe? Could be slipping. That might fit, but it would show a slight speed drop unless it happens very quickly. This can be oil and adjustment related. I won't tell you what oil to use, but you can read up if you're unsure you're using a good one for wet clutches.
Or intermittent spark. For the spark, get the bike good and warm and see if it still has the problem standing still. Ignition coils will give a similar effect that gets worse as the problem goes on, eventually feeling like low fuel, but it only happens when it's hot usually and in high revs.
As well, possibly a jet in the carb, but if it's hard to recreate the issue I would lean away from that one.