If the problem hits while downshifting,, It is probably not just the cable since you are pulling the cable under a load from the shifter and it is a direct movement. If the cable was sticking, it would hesitate or not shift when shifting up. I had a similar problem with a campy 10 speed. I could hit every gear but it skipped back and forth on the center two gears. I had to re-time the entire cassette including opening up the limit screws. It was happening because the shifters were one detent off the way we had the timing set.
Try to reset your derailleur from the beginning and see if you can hit all the gears.