[rodw]

rodw: Can you help me understand something, maybe I am mistaken. I assumed that a PSU could generate EMI due to it's internal circuitry, my suspicion was this was causing problems. Would an EMI filter address that?
And the benefit of the EMI filter is that it would filter out EMI from the mains? Does this EMI pass into the electric components through wiring or from radiowaves emitted from the mains wires?

I'm no expert but noise is a funny thing. It might be a bit different for you as I was working on a plasma cutter and I was getting a noisy arc voltage signal. I bought a cheap USB oscilloscope to investigate. I concluded the bulk of my noise was on the mains power supply (you can still see an AC ripple in the DC supply). So I added an EMI filter (just replaced the IEC connector with a filtered one). Progressively, I removed some of the work arounds I had added. The only other thing I have done is add separate earth wires from every motor case back to a star ground point so there is a path over any linear rails.

So by far the biggest improvement came from the IEC filter. For $10 its cheap insurance!