Bob's nearly hit the process on it's head, nearly. I suggest to separate the various aspects of a system when accessing it. Move the der independent of the cable or lever. Move the cable then the control independent of the rest. Set your der initial adjustments with out the lever of cable being used. Then move onto the cable being pulled mid way along it's length to shift the der. Only use the control/lever when all the rest is working well.

Other aspects of a bike benefit from this same process. Understand what is first needed for the second aspect to be able to be looked at. Example are those who true wheels but don't bother with a bearing adjustment first. Wrong order. Do the bearing work first as how the hub works can affect how the rim looks. Andy