I'd suspect the VFD. Your video doesn't show it, but there must be one, right? Sometimes they can lose their settings. Have you gone through all the parameters and made sure they're all set right? If the wiring is all tightly connected? (It's acting like something is loose and conducting intermittently) If there's nothing you can find wrong, replacing the VFD might be worth trying. If it behaves the same way with a new one, then replacing the spindle would be next.