Hey Kendell,
the settings are ‘drive-current’ and ‘idle-current’ on the Motor 0 page. Also adjusting the ‘stall-current’ may be useful. You may search for these terms in conjunction with “wall” or “vertical”. I have no experience with a CNC mounted vertically, but some people here have.
But since the machine works correctly most of the time, I would check the possible causes mentioned in the document linked above one by one.
To check the cable, I would unplug it on the controller side and on the other end and check all four wires one by one for conductivity between both ends, by using a multimeter set to resistance (ohms). But it could be that you measure that everything is okay because the fault my appear only in certain moments, in certain positions of the cable, when you move it, etc. Usually it’s the motion on a cable that is not made for permanent motion that can make problems after a while, mostly where the individual wire is crimped into the contact, or between the contacts which unfortunately are only tin-plated which means movement and/or corrosion can wear the plating leaving the bare copper open to air which is even more prone to corrosion. In this case, a little amount of KONTAKT 60 and then KONTAKT 61 can help.