We encountered a problem with motors were they break and they spin in different directions at different speeds.
Does it require significantly more force to spin the motor by hand than other motors.
Replace the motor with another just in the wiring and see if it spins in your hand at different speeds.
Are you using any elastic assistance to help take the load off of the motors? Just because the motors can lift the arm with your gear ratio doesn’t mean they will after several minutes of use. At some point that thermal breaker will probably trip and using elastic assistance will help extend that period.
We were experimenting with elastic when we first observed the problem. We had tensioned the elastic such that the arm stayed up on its own with no sacks, but did not raise on its own. The arm is 15" long, and has 2 393s geared 7:1. It was not loaded with sacks when we observed this issue.
We didn’t try back-driving the motor by hand, but we’ll try those experiments at our next meeting. Thanks for the suggestions!