My micro P51 had a run in with a curb, nose first on the roll out of a landing. After that, the motor would run for just a little while, then start skipping and eventually, sometimes, stop completely. I thought the batteries were bad, they weren't, I took it apart and fiddled with it, the motor turned over ok, and the shaft wasn't bent. Drove me crazy. If I remember right, (this was about 3 years ago) there was a little red LED inside the airplane that would blink when the motor stopped. I could unplug the battery and reset everything, and it would come on again, but usually just long enough to get airborne then start again. Anyway, long story short, I bought a replacement motor, and it's still working. I think the little brushes inside the motor got jarred loose or something. Anyway, sounds like what you are describing, only sometimes mine would stop completely.
Oh yeah, when I would apply battery voltage to the motor wires directly the motor would run, making me think the problem was in the circuit board. My dad convinced me to by a new motor for it. I guess the motor would draw too much current or voltage or both, maybe that was messing with the speed control cut-off?