Only thing in software likely to cause problem is Configuration which you can set, if you want to check and find problem swap over X and Z axis motors and move carefully if X stops working and Z works its either the wiring or configuration in Mach3 as motor etc fine then you could swap back and just swap driver over to check Driver ok, I would first of all post your Input settings For XY&Z and current Motor tuning for X and Z axis as it may well be obvious from those what your problem is.