I would guess the problem to be the slipper. The pinions (smaller gears) attached to the motors spin the spur (large) gear. The spur gear is not coupled to the transmission drive shaft and should not be. There are two plates that press against the spur gear that is intended to provide an upper limit of torque that the motors can put out through the drive train. The amount of friction is controlled by a spring on the shaft that is secured by a lock nut. Tighten that nut a little bit and see if the problem persists.

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