This is an issue that has been addressed a number of times. The design of the many models of lower cost plasma cutters like the cut40, lotos etc. simply aren't designed to be used with CNC machines. The levels and types of EMF simply kill the PC. I have tried and read of other folks trials to eliminate the issues. You don't say which driver you are using etc, but grounding, shielding etc., etc., seem to be of little to no use with some drivers and breakout boards (G540 or example). A little research will point you to a couple of threads on this forum which may offer a little more insight.