This is very common with compressors, esp with USB, back in the stone age long before Ethernet controllers where common I had the same problem with one of the first USB Smooth steepers from Warp9. My compressor was outside on the opposite side of the wall in the compressor house however it shared the same ring main as the PC. The fix was to hack the cable shield which was causing a ground loop and lot of messing around, but even then it was never fully stable. I wouldn't ever use USB for a CNC controller as it's far too unstable.

Regards UCCNC or MACH3 then I'd go with UCCNC. As most on here know I've been a long-time user of Mach3 and have built or retrofit fitted 100's of machines that use it and various controllers that work with it. However, I've recently made the decision to soley use UCCNC and AXBB-E or UC300ETH/UCBB boards on all the machines I build from now on. The reason for this decision is because Mach3 is no longer supported and Mach4 is full of bugs, Also the controllers are very well built and the UCCNC software just gets bettter and better.

Now I was going to create a thread to say this but I'll mention it here in case it helps you. I'm buying the AXBB-E and UC300ETH controllers in bulk for the machines I build/convert and I'm happy to sell to forum members to save them the hassle of importing etc. I'm also doing the same with Lichuan Closed loop steppers and drives.

If this interests you then get in touch via PM.