Ok well first why are you using USB when the PLCM is capable of using Ethernet.? Ethernet is much much better at handling noise than USB.
Need some idea or pics of how you have wiring/earths etc because you shouldn't be having PC earth interfering with Control box earth and vise versa. You haven't earhed the PC to control box earth have you.? PC should just use the Earth in the Plug and no external earth to control or machine frame.
Also have you got a Earth from machine frame to Control Star earth.? If not then do so.

Can you post your Mach3 XML file (Zip it up) it will be in mach3 folder using same name as the profile your loading from Mach3 loader.

What VFD is it and does it work normally Ie: ~using buttons on front.?
What settings have you changed in the VFD and what did you enter.?
What inputs on the VFD are you connecting to.?