is it squealing like a fan belt does on a car when it slips,this is common on scooters that have been made to go faster with big bore kits ect..i would personally fit a malossi kevlar belt to it,also check the variator hlfa for wear they are aluminium but are plated with nickel and if it rubs off then they will need replacing to as it will not operate right and the gearing will be wrong and the belt will slip.
adam bike doctor, February 2010