Try to determine which cable is bringing the RF into the device. If it’s not the USB cable it may require trying to isolate and bypass the interconnection used by the various services that the Navigator is providing such as CAT control, PTT, Sound Card services or FSK and CW keying if you are using it for that. Once you’ve determined how the RF is getting into it you will be able to choke it off effectively by experimenting with your torroids. I don’t think that grounding will be the solution.
Good luck! QRO is nice but it comes with headaches. It took me forever to address all the devices in my house that would lose their mind when I ran power.
Some say that a high quality balun at the antenna’s feed point works wonders, but in my case, due to the complexity and hi-q of my Pro-67B’s feed point installing a balun wasn’t practical.