I had a full dura ace set up and I was getting sloppy shifts in the middle of my cassette. My LBS said that I may have worn out a couple of my favorite cogs. It made sense so I repaced my 12- 27 with an 11-28 Ultegra cassette. I made some minor indexing adjustments and adjusted the B-angle to clear the 28 tooth cog. It still was very hesitant shifting from 14 to 13. All the other shifts are acceptably crisp. They repaced the cable, but that one shift is very iffy. Why would that one shift be a problem?