The Z bearings require Lithium grease, I have been having this issue on again off again for over a year. My Z bearings feel fairly terrible sticky and gravely. I am going to try to clean them out with WD-40, then re-grease with white lithium grease. Will let you all know my results. The bearing info was found at: https://us.misumi-ec.com/pdf/fa/2012/p1_0265.pdf Hears to hoping this solves my issues!