It sounds like a fuel delivery problem to me. Your bike is requiring more fuel at the higher rpms, and it just isn't getting the fuel it needs. Did you check your petcock and make sure it's set to on and not knocked somewhere in between? That's the easiest thing to check. Beyond that, the problem could really be anywhere from the petcock to the cylinder, though I would imagine the problem is somewhere in the fuel lines running to the carburetor or the carburetor itself. Also, check and make sure there aren't any vacuum lines hanging down or anything.