What is wrong with my scooter?

I bought a used 2008 Vento Triton. It ran perfectly fine for the first few days then all of a sudden whenever I start it up it will start fine then after getting to about 10mph it sputters and dies. I can let it idle for a long time and it sounds fine but once I hit 10mph, everytime it will sputter and die. The gas is full and the oil is filled as well so I have no idea what's wrong with it. Help?