Quote Originally Posted by Clive S View Post
Then that sounds like EMI
Clive is spot on here.
The EMI often tracks up the mains lead from the spindle motor/driver and back down the lead to the PSU for the control system, separating the two to plug-in points opposite ends of the ring main can help, or running the spindle motor/driver from the furthest possible socket via an extension lead.
Ideally you'd fit separate EMI filters to the mains inputs close to the equipment.