1.visually check wiring and connections on table. unplug and plug connections
2 swap the z and x or y geckos and see if it is a gecko issue.
3 Swap the x and z plugs on the table you might need to swap the same mach3 settings when doing this. then just make the z axis run up and down using the x axis controls
4/last thing would be to disconnect the z stepper and run the z axis up and down by hand to make sure you are not binding anywhere.
they dont need to be in this order. you can also change the accel rates and max speed of the z in mach3 and see if this helps issue.