Taking a guess at it, the esc is acting up, but depends on how much you ran this set up, and how hot you got the motor, for me personally I only run down to a 10.5 motor, as I tend to get carried away with the power and speed of the bigger motors and tend to over heat them, by just screwing around either on the track or in front of the house, but honestly the only way to determine which one is at fault is to change one and/or the other so if you have a spare motor, change it same goes for the esc

By chance have you tried to recalibrate the esc??