Thanks for the response, Ryan!
I turned up the pot until the lead screw turns without chattering. Now the problem is just that it is inconsistent in its direction. For example, I turn the knob CCW once, and the screw turns CCW. Turn the knob another click CCW, and this time the screw turns CW. Another click, and the screw turns CW part way, and then CCW. I can’t see any rhyme or reason to how it decides which direction to go. I plugged the X axis into the Z axis on the board, and it controlled just fine, with no unpredictable behavior. Any ideas?