You plasma is isolated from the table? Or do you mean the electronics (motor drives and PC logic) are isolated. If your drives don't have isolation on the Step & Dir and you don't ground things perfectly you can get surges from the HF palsma that kills logic level electronics. Using a HF plasma with a system designed for plasma (where isolation is planned and done in every part of the electronics) is often a challenge from a noise perspective but bad grounding can destroy drives and even your PC if you don't have isolated inputs.
I fear you are trying to apply a hobby level solution to an industrial cutting problem.