My first instinct when there's sloppy shifting in the middle of the cassette is to look for sticky cable movement. Dirt, rust, fraying of the wire at the lever or thickened, dry grease in the housings can make the cable return slow or incomplete on upshifts, making accurate trim impossible.
My definitive test is to do a down shift, followed by a 1 gear upshift, and expect poor response. Then firmly pluck the bare wire away from the downtube like a guitar string. The extra tension of the pluck pulls down any hung up wire, and if that corrects the trim you know it's cable related and can proceed to fieldstrip the cable and inspect, relube or replace as necessary.
BTW- if this was a fairly rapid onset, take the extra time to remove the lever and inspect the wire at the lever. Very often a fraying wire will cause sloppy shifting before it fails completely. I was reminded of this on my commuter, which started shifting a bit worse, and I decided to put off checking things until I had a bit more time, but the bike had other ideas and I rode home in high gear Tuesday when the cable broke.
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FB
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