A lot of window motors are thermally regulated. When they get too hot there is an internal switch that will trip, cutting off power to the motor. When the motor cools down that switch closes again restoring power to the motor. You said you replaced the motor with a used one. It could very well be that the used one is bad as well. You may want to try lubricating your window track in the door with silicone paste. This will make the window slide much easier and reduce how hard the motor has to work. This may solve your problem, otherwise it sounds like you might need a new motor.