When my Cnc plasma cutting pierces the arc sometimes working good and sometimes no, good main the arc on and off time working at right time, but sometimes ( off ) not working right and late to end arc, machine go to the next Pierce and arc still on , l change relay in THC card with new relay but still arc on right and arc off is late, I tried to connect the arc tourch direct with the lgk power source without THC card also still the problem arc on right but arc off late, how can I fix it please