[mactec54]

I'm absolutely certain it is noise - but is the noise causing a blip on the e-stop? On a limit switch input? Causing the USB controller to reset?

It seems like there has to be a way to see WHAT is causing the trip, somewhere in a log file.



No reactor/power filter on the VFD - but the VFD is fed from a completely separate 220V sub panel than the motors/controllers.



It's bog standard 14/4 SJT. There really is nothing to go wrong unless the wire is damaged in some way. It's been inspected and the insulation is in good shape with no evidence of damage.



Wiring from sub panel to VFD, and VFD to spindle motor is unshielded 14/4 SJT.

You can not use a VFD Drive without a Shielded cable from the VFD Drive to the Motor, that being said it can be done with twisted wires and put inside a conduit that has a metal lining, this acts like a shield it must be Grounded though

You should also have a EMI Power Filter before the VFD on the Input Power supply

If you have a separate sub panel this is not a good idea, as you most likely have a Ground loop, by doing this setup You can only have ( 1 ) Ground source

USB cables can be a problem, if it is double shielded with ferrites at each end you should not have a problem with that

So it looks like you have a lot of possible problems