I would recommend doing the initialization procedure whatever it is since it is electronic fuel injection. The sequence should be in the service manual or lacking that maybe the dealer will share from his. This will initialize the TPS (throttle position sensor) and the ISP ( idle speed control). This seems to solve nearly all the idle, surging dropping dead at the traffic light problems in the other injected Kymco models. Let us know what you find.
By the way, my dealer told me he has yet to get a complaint on the DT300i of poor idle, surging or dying. This tells me that either no one else has had these problems, he has dealt with them like your dealer and/or his techs do not know the procedure or the service manual.
Karl