Campagnolo Ergopower inaccurate shifting on high and low cogs
The problem is that you have the cable incorrectly routed under the pinch bolt / washer.

You need to route the cable down the same "side" of the pinch bolt, but the washer needs to turn 90 degrees clockwise, so that one "claw" sits in the hole in the derailleur body, one "claw" sits outside the derailleur body next to the free end of the cable and the cable itself should be trapped, in a straight line, under the small extended flange on the washer that has a groove cut in it, for that purpose.

You might need to use a new cable as the way you have it clamped, it's forced a "kink" into it which will probably not straighten out fully and so will always affect the cable tension, slightly, so giving you inaccurate shifting.