If you are running 10-15 minutes STRAIGHT, without letting things cool off, you are killing the motor...
Borrow a temp gun from a nitro racer and check the temp of the motor. you normally want to keep it at or below 170 degrees. The worst I've ever seen was a stock sedan motor that came off the track at 230 degrees after a 5 minute race -- the motor was never the same (magnets lose strength from heat, etc.)...