Ok...time for one of my Tony Hart drawings ;)

This is what I was trying to say....to me it looks as though you have the gantry over the front rail, this causes a large overhang of the Z axis and is far away from the bearings. Top scribble.

If you move the gantry so it sits over the back bearing there will be less of an overhang and it will be better. Bottom scribble.

Click image for larger version. 

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