Peugeot Speedfight 2 speed very high but only after some time starts??? help

Hello I have a Peugeot Speedfight 2 that starts and the speed is normal at the beginning, but if I want to get on it and start driving, the speed goes very high until it starts slowly and the speed stays up and when I turn off the scooter but let it run without giving gas it goes night for some time you hear right how the speed goes down? What is the reason for this how can I change it?