it sounds like the final drive gears have broken up,they are in a case at the back of the engine.its really hard to say exactly what it is but it sounds like this to me .i have seen a few scooters do it but generally its italian scooters that have this problem not french ones.you need to take the left hand transmission cover off (the one the kickstart lever goes into) and start the bike with it off and see if the belt is driving the clutch around at the back of the engine,if it is then it is definately going to be the final drive problem.
adam bike doctor
September 2006