From your description of the problem, it sounds like the problem could possibly be with the battery pack. We would begin diagnosis with a simple test involving propping the front of the cart in the air so its powered wheel is off of the ground and can spin freely. Then see if the motor runs normally when the pedal is pressed. If it does then that points towards the battery pack dropping too much Voltage under load. Please let us know how it goes.
Erik R Wilkin