Are you following good grounding practice, i.e. service ground and any machine grounds to a star point and all metal parts of the the machine and spindle frame with ground conductor?
Are you running isolated power supplies?
Do you have a DeskTop PC or laptop?
If DT. PC. how and what manner of grounds are connected to the PC power supply?
Personally I prefer to make an absolute common ground connection of ALL power supplies etc to the common ground point. i.e. nothing is isolated.