Bob McEnaney gave me an idea for another t.o.t.w. on last Sat. ride. When we were riding side by side, he said his bike wasn't shifting very well. I suggested that we stop and turn his rear derailleur adjuster half a turn counter clockwise. That did the trick! The problem is that shifter cables stretch out, especially if it is new. One way to avoid some of that is to pre-stress the new cable once it is on the bike. Grab the cable with your fingers at the chain stays and pull it away from the stays. That will stretch it out a little, but you will probably have to use the barrel adjuster for a fine adjustment later on when the cable stretches out again. Usually that will be the end of the stretching out. Just remember counter clockwise, and just a half turn at a time.