Did you try loosening the middle screw? That will lock the axis if it's tight. Other problems could include one of the bolts that holds the Z axis assembly to the column working loose and interfering with motion, or the nut on the top of the leadscrew being too loose, so that the leadscrew flops out of position. If you've adjusted the bronze nut that transmits motion from the screw to the axis, you might have got it too tight - try loosening that a bit if none of these other suggestions fixes the problem.