My 50cc scooter keeps cutting out?

I have a Jinlun 50cc ninja scooter which keeps cutting out. It is only a year old and has just been through a service. It starts fine, runs for about 30 seconds then the revs drop and no power, then it stalls. I restart it and it goes for about a minute and does the same thing. After half a dozen attempts it seems to be back to normal, albeit with slightly less power. Any suggestions what this is?