That is giving your stepper 2A, if you are not using my steppers they probably can’t handle that for long. I run them at .7V max. You will need active cooling as well or your driver will go into thermal shutdown.
Now for your problem. What version Z axis are you using? No, anti backlash correct? The z axis should move extremely easily no signs of binding what so ever. You have binding somewhere, Probably crappy to hear but You are probably going to have to take the center and Zaxis apart and do it again. Make sure everything is aligned properly.