Check to see if the ball screw is loose. There should be a bearing nut to control thrust and it could come loose. Also check any other mounting bolts for any other parts in the axis and the shaft coupler for movement or loose fasteners.
This answer is a little late but slowly losing position often indicates an encoder problem. The problem could be the encoder itself but is more likely electrical noise getting to the encoder signals. It can also be that one encoder line is not switching. If one of the differential signals is not switching the count may appear to be working but counts will be missed.
