The power of the motor is not what is causing the problem. If you are running pin tires on carpet (basically 0 slip), something has to give. Even with a 17.5, when the throttle is applied in the air and the car lands, the rotation momentum of the drive train must be absorbed. Without a slipper or tires/racing surface that gives during landing, the next weakest link will fail and in this case, that appears to be the idler gear.