Stepper moves in random directions (forward or backwards)
 

When running G-code (or jogging) the stepper will either move in the correct direction or in the opposite direction, randomly. I've replaced the Gecko drive and the wires to it (some looked pretty cooked). I even replaced the entire PC that I use to run the G-code; I made a clone as a spare, when I set up the machine.

Before and after all the attempts at repair, the malfunction happens exactly the same, without the slightest change. I don't know what to try next.

Is it just on one axis?

If you swap either the motor, or driver with another one does it keep doing it on the same one, or on the swapped one?