Rear derailleur shifts all gears at once I've got Campagnolo Veloce gear system and I recently changed my worn-out rear cassette to a new one. Since then I've experienced issues when shifting to higher gears. When I shift, the cable moves all the way to its extreme position, or at least skips 3 cogs. Shifting to lower gears works just fine. This issue is likely not related to the pulley/cassette because I can feel the shifter releasing way too much even if the cable is not connected to rear derailleur.

I have done the indexing and H, L and B screw tuning. Do you have any suggestions how to fix this?

EDIT: The chains were also changed at the same time with the cassette.
Your problem is worn out (or broken) index springs aka G-springs. These are part of the shifter mechanism. Worn out G-springs will enable rear derailleur spring to pull the cable over the indexes. 

One way to test is putting the rear derailleur on bigger sprocket and pull the cable. If it drops indexes (goes to smaller sprocket) then the problem is caused by G-springs.

G-springs are replaceable in older (pre 2007) Campagnolo shifters. Only tools needed are hex keys.