My new Knight Hauler/Freightliner

[mavrick0]

Sounds as if you are overheating the ESC and it is going into thermal protection mode and shutting things down. Without the trailer there isn't the load on the ESC so there is no reason for it overheat. Not knowing what trailer you are running I'm going to assume it's probably heavier then the stock Tamiya trailer. One suggestion would be to change the pinion on the motor and drop it down a tooth or two or try going to a crawler motor that has more torque for pulling but not as fast as the stock motor.