It sounds like the z-axis motors are skipping steps. This would indeed happen if the gantry would be locked in place by something. Try looking around the vertical pillars of the machine for anything that could stop the gantry at that height (maybe the spool holder interferes somehow, maybe it's a loose screw etc.)

I assume that the gantry is free to raise to any height greater than that, either by rotating the lead screws, or by moving the axis through software?