My Suzuki Katana 750 starts to cut out after it warms up?

I have a 1998 Suzuki Katana GSX 750. When it is cold it runs like a demon. When it warms up after about 10-15 min. It starts to cut out. It does not die it just gets really sluggish. When warmed up it still idle's fine but when I give it throttle as stated before it just starts to lag really bad until I get up to speed then it evens out as long as I don't get on it. I had the carb's cleaned, oil changed, air filter changed and the plugs changed. It had this problem before the work was done and still has the problem. Also I don't know if this is any help but the guy that had the bike before me had put on a new Vans & Hines pipe and the bike was not re-jeted so I don't know if this would cause the symptoms or not. Any educated opinions would be great. Thanks