Increase stiffness of Z axis


I am looking for way to increase the stiffness of the Z-Axis when milling more than about 7cm from the middle assembly. In the range from 0-7cm below the center the accuracy is really good, but going further down to more than 10cm from the middle assembly, circles tends to get a ditch in SE quadrant and to carve more then expected in the SW quadrant.

Any ideas about this?

Take your z axis off, leave the tool mounted, lay the tubes on a flat surface like a glass mirror. Do they both touch nicely or are they twisted?

I set the tubes on a glass desk first with the tool holder then put it together and it usually stay straight. Hard to explain hope that makes sense.