I put a new cable on mine a couple of months back and found that I was having the same issue. I adjusted and readjusted and re-ran the cable and effed with it for days but couldn't get it right. Out of sheer frustration I took it to my bike shop who looked at it and in 2 seconds told me what the problem was: I'd secured the cable on the wrong side of the, eh, nut/bolt thing, which changed the pull on the derailleur.

It's a dumb mistake, but make sure that's not it.