I still don't know what happened, but I'm not going to complain about it anymore. There is very little doubt I was abusing the scooter by overloading it and riding it at WOT 95% of the time. It's a stock setup, but yes I did let it run too fast downhill at full throttle several times. I also had the habit of going from WOT to fully closed throttle, and it looks like that might cause the belt to bunch up on the front pulley. I didn't realize until after this happened just how loose the belt was on the pulleys. A chain running anywhere near that loose would either come off the sprocket of jerk around and break. I plan on selling this scooter fairly soon, and getting something bigger, that won't have to be run at full throttle constantly. I am still at a loss to explain why the first 2 belts lasted so long, being treated the same way, but I've seen stranger things.