Dont see any mention of isolation there so probably not.
As for your last question, measuring at the port or cable end would determine whether the problem starts there, ie noise on the cable or a bad port but as it is now random and you would have the motors disconnected it would be hard to know if it should have changed as the DROs in Mach show correctly. You will probably not even see it with a voltmeter if its noise, a scope would be needed, so probably best just concentrating on the outputs of the BOB for the time being now we have determined Mach is acting correctly.
Hood
Hood