Strange.. A couple things I can think of off the top of my head

- maybe the pinion gear isn't set tight enough? Maybe the high rpm's make it slip off the shaft so the shaft simply spins at super high-rpm's?

- maybe the slipper is too loose and it starts slipping at the high-rpm's?

- Maybe the motor has gone bad? I had a problem very similar to that before.. but in a crappy brushed motor. I don't know if the problems relate or not, but it's a thought.