You know, it might be that the ESC is overheating. The manual says:
Heat protection mechanism: if TEU-101BK starts to overheat due to long use, power to motor is reduced, causing car to slow. If further overheating occurs, power to motor is stopped, preventing any damage. After cooling, the heat protection mechanism will automatically reset.
Is this happening after you've run the car for a long time? I noticed it happening on a buddy's car after he had been running his car (with a 27 turn motor) for about 10 minutes.