Soundes like you may have a little slack in the cable. Sometimes as a cable stretches, you first notice shifting irregularities in the middle cogs. Shift the rear derailleur to the smallest cog and check for slack. If you have some,turn the barrel adjuster on the rear derailleur counter-clockwise about 1/4 turn, and ride-test to see if that fixes your shift problem. It may take a couple times of small adjustments to get it right, but I caution about large adjustments, which can cause shifting to change for the worse. Fine tuning takes patience, but is rewarded when done properly.