I've been fighting this issue as well. Here are the thoughts I've gotten elsewhere:
1) Check the air filter
2) Drain the carb bowls
3) Pull the carbs and do a full cleaning
I've gotten 1 done with no improvment and I'm in the midst of metering in a fuel treatment in hopes the blockage (assuming there is some) is minor and can be cleared while running the bike. I dread 3 only b/c getting my carbs on and off is a real bitch. That won't happen until there's snow on the ground, that's for certain.
I believe for me the reason its an issue at around about those rpms is that's the point at which one of the jets kicks in. Just supposition and I haven't really thought through that process yet. There's a possibility the person before me did some jetting but didn't use a good kit too, but I doubt that's an issue for either of you.
Dragging knee is for the track, and dragging tail is for the lot. --Kane Friesen
When you're in a car, you're watching a movie; when you're on a bike, you're in the movie. --Robert Pirsig
Identity theft is not Fun