Machine adding steps when milling

Hi, my cnc router as started to add steps on the z axis when machining. I am cutting aluminium profile at 3mm deep slots. After two passes, the machine as added on 6mm on the z axle. I have checked the connection between motor and ball screw and that’s ok, I have checked that the mill end is not pulling out, that’s ok. Anyone have any ideas what is going on. I have tried to restart machine, pc etc, but it still happens. However if I am air cutting it does not. Any one know what’s wrong.