Ok if my understanding is right, you have changed the cable 3 times already so I think we can eliminate conductor breaks or other issues such as that. You have checked the equipment so you know that the devices can communicate. Obviously you must be going by some problematic noise sources. Have you maintained some separation from power cabling and other noise sources? Do you cross over noise producing cables perpendicularly to limit noise? There are always other options to look at, but you may spend a considerable amount of time tracking down where noise is being injected. If you have not already, you may want to turn off that DC drive and see if your problems go away. If you have other devices that you can do that with, you may at least determine where the problem is coming from. After all that you may still need to install shielded cable. That may actually be the quickest solution to your problem.