Help diganosing fault with chinese Chinese CNC router

[JohnHaine]

Two possible causes occur to me. One is that there is an intermittent connection in the z direction wire, but you seem to have ruled that out. The other is that the setup of when the direction signal changes relative to the step pulse is wrong. If they are coincident then the driver can get confused over which way to move. Without detailed information on the controller I can't comment on whether there is a configuration option for this.

Edit: I found the manual in case you don't have a copy. On a quick read I can't find an obvious option to change this.