Today i cured all my problems by connecting my VFD ground directly to metal box with electronics inside. Incredible but yes.

As i read that Dean suggested no ground from box to PC, i remembered that when was desperate i did exactly that. Took off ground from PC side and spindle started rising and slowing speed. I said to myself that things are getting worse like that, but as the ground to box was still connected i started touching it around. Touched to ground on socket and was better, touched to PC box- better, then accidentally touched to VFD ground and voila, all was fine. They have common ground at socket, but cable between boxes cured all problems.