98 GSXR600 not running well, need help.

My GSXR600(carb) is not running good. It runs good except when I give it throttle really fast, it stumbles, or hesitates. It also hesitates when I shift up. If I shift really fast it seems to be minimal. When I accellerate slowly there is no problem. It has 9700 miles on it. It is stock except for a D&D slip on and a K&N air filter(factory style). I took the carbs off and opened one up, it looked REALLY clean. There is a little Valve niose, but it has been there for a long time, and it just recently started acting up. I had a mechanic at the dealer listen to it and he said it wasn't bad. I changed the spark plugs, installed the new K&N air filter, and it seemed to get worse. Can someone please help me?