Quote Originally Posted by RichR View Post
Plugging into a long extension from another room and the problem disappears
A long extension simply helps to attenuate high frequency energy - the same can be had with a filtered inlet (e.g. IEC chassis filtered plug), and/or possibly including ferrites on the various cables. But the former means modifications to the control box. You can get filtered extensions that might help, but I'd trust the IEC filter first. To be fair, the usual photos of the control box innards tend to show large filters as part of the design.

What's interesting from your descriptions of the problem is that this sounds to be a logic-level issue, either the BoB interface or the stepper / opto interfaces. This is either a poorly design board, really poor PSU, or some fault on board (there shouldn't be any substantial influence from the spindle). One thing to consider is that the spindle under load is likely to produce more EM noise than unloaded.