This is fine. Factory recommendation is also overall-shielded-cables. I have no problem using overall-shielded twisted-pair.The cable is twisted pair but it is overall shielding
My suspicion is that some of soldering might have gone bad or short-circuited.
As there was no alarm as you said, nothing can be definitely said. Just an honest guess.
Cold-soldering or bad-contact can make random behaviors. If position-feedback is incorrect, I am sure amp can not push output current at right timing, then output torque might be going up and down randomly.On the otherhand why would it work perfectly sometimes and not the other? Also does the cable configuration really affect the torque output?
All are based on assumption.
However, symptoms are consistent through motors and amps then there should be common components.
I guess it would be cable-connectors.
regards,
Kevin