Although it does sound like a limiter screw issue, how old is the lever? I know it is 9-spd but I mean old in terms of use? Do you have a ton of miles on it or is it still in fairly new condition? You should replace the G-Springs every few years.
Are all the housings seated well? Did you cut the housing that runs from the chain stay to the der or did you leave it at the original length? Campagnolo ders like that cable to be a good bit longer than Shimano does. Having this cable too short can cause this problem.
Are you using Campagnolo cables and housings? Campagnolo cable housings are much stiffer than Shimano or Jaqwire. Soft housings can also cause this problem.
Have a look at your screw. If adjusting that doesn't do the trick, move on to these low-cost options.