On my x3 i have a problem with the z axis at the top few inches - does the same thing. I noticed before I converted it - the handle would turn easy at one point and then get hard to turn the next time around. It would only do this the last few inches. Evidently the end of the shaft was not turned concentric to the threads of the screw so at the point where it lined up correctly the handle would turn easy and then when it got too far on the low side or high side of the screw it would tighten up. I still haven't got this fixed so I have to set the stepper to a slower speed than normal to get it to go all the way to the top. You may have a similar problem. I see ball screws in my near future yet.