What breakout boards, etc. do you have between your PC and the spindle?
Everyone's idea of "properly grounded" is different. Can you describe (or draw) your grounding scheme.
Make sure the power wiring to the inverter and spindle is as far away from all other wiring as physically possible. And don't have long parallel runs of the power spindle power wiring and any limit/home switch wiring. Even if the cables are shielded, the switching current in the spindle inverter and resulting electrical noise can radiate all over the place.
