I would suggest that you find a qualified TECH in your area that could go over the machine and check it out for you. At least he could write down what items are in use on your machine such as THC make and model, drives and motors. AND check the setup for the motor parameters, Check for binding on the Z axis ,Check the % of rapid setting on the THC controls, Check the Zheight switchfunction, LOOK for noisy wiring configurations and ground loops ,etc,etc
(;-)TP