stage coax controller arm moving stage diagonally, not straight line forward and backward

I have an older VWR inverted fluorescence scope with an installed aftermarket stage with coaxial movement controller handle arm. No idea what brand. The controller moves stage toward me in a straight line but when i'm moving the stage away from me it moves in a diagonal fashion and I can't keep a straight line when trying to look at slide or 96-well plate. It's drifting toward to the left and another column each time. Has anyone had this issue? Is it in the controller arm and can it be taken apart, repaired? I fear that the arm is completely tied in to the aftermarket stage and would be an expensive replacement. Thanks, MPi

Equipment: VWR inverted scope 89404-464