Plasma cutter CNC

I've built a CNC machine and it works really well with my router, when ever I go to use y plasma cutter I "kill my arduino R3, I have tracked down the source of the problem (I think) it appears that when I cease cutting, it is "spiking the power to my MAC which is passing this on to the Arduino board exceeding the 5v max. does anyone have any ideas or suggestions?
thanks

It's a bit hard to see your workshop from here.

What is a "MAC"?

Post a circuit diagram.

...R

Firstly if the power spiked that badly in your Mac it would likely be dead...

Secondly you have a very high EMI enviroment I suspect, lots of high power electric cables running
around blasting out interference. It is possible the interference is getting into the Arduino by various
routes, without a good overview of how everything is connected and which wires run near the sources
of interference, its impossible to say what the (main) problem is.

First question, do you have a power-conditioner on the mains supply to the Mac?