I had to install a larger, double torque stepper motor to control the Z-Axis which solved the problem. The original one was just too small. I also added, just for good measure, a 1" diameter spring on the outside to help hold the spindle. Since the modification, absolutely no problems. On a newer machine I bought, I had it equipped with the larger stepper motor, but with air cylinders to help for the weight of the spindle. Again, no problems.