Have you checked out the engine temp sensor, as you bike might be getting a cold motor signal which would be fine while motor is cold but once warmed up if faulty might be telling ECU that motor is still cold which would cause the ECU to enrich the fuel and raise the idle speed which also could cause the spluttering due to much fuel
I would be checking the connection to temp sensor before getting it check out by dealer, with its age the sensor could have gone bad