I am not saying you should actually do this, but I once had a problem that was similar, I could not tell if the problem was mechanical or in the drive, I rode the car with the power off and had my helper lift the Brake by hand so it would drift in the up direction. Of course he dropped the Brake before it over sped. Did you replace the velocity encoder? does it drop the Brake when it stops and starts? Is this a geared machine? Are you registering any drive faults? Have you performed an auto tune? Sometimes a power module in the drive will be flaky and cause this type of problem, an auto tune will identify a bad one. Is the position encoder on the car top or the Governor, if its on the gov. check the rope and all the linkage, as well as the encoder coupling, and the velocity encoder coupling. This sounds like its a geared machine. Is it? what is the FPM of the unit.
