DC Motor power diminishes quickly
Increased temperature of the motor windings will increase the winding resistance and reduce the maximum torque that can be produced with a given applied voltage. You could compensate by applying a higher voltage, but that may cause the motor to be damaged. You probably need a larger motor. Along with a larger motor, you should use a motor controller that will limit the motor current electronically. The current needs to be limited to a level that will produce the required torque but not damage the motor with the operation vs. rest duty cycle that you need. It may be difficult to find a motor with a published duty cycle rating.