As Jazz says you need to use shielded cabling with everything. The HF pulse from plasma start will go anywhere and everywhere possible if you dont get the grounding correct.
Ground your shielded cables at one end only, preferably the supply end and get all of your ground cabling back to a single star point on the machine somewhere then use a single large cross sectional area conductor from here to ground. You could even try using an earth electrode outside your workshop hammered into the ground and connecting your star point to this.
Short story but this got a customers machine working years ago doing this. His machine was working fine until he had new building work done. A reinforcing concrete wall was built at the end of his workshop and this had foundations that were alot deeper than the original. This dried out the ground under his workshop and prevented a good earth from his building steel work that his machine star point was connected to. Basically it lowered the water table level. I hammered in a TT rod ouside his workshop and connected the star point to this and the machine work as it should without jumping all over the place when the HF kicked in.