Joined: Mar 2014
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From: ,location, location
Bikes: Foundry Chilkoot Ti W/Ultegra Di2, Salsa Timberjack Ti, Cinelli Mash Work RandoCross Fun Time Machine, 1x9 XT Parts Hybrid, Co-Motion Cascadia, Specialized Langster, Phil Wood Apple VeloXS Frame (w/DA 7400), R+M Supercharger2 Rohloff, Habanero Ti 26
If the cassette is used it is probably worn out at least in that cog, I would just toss it and get something new. There is no more troubleshooting needed it is an old used cassette of unknown usage. XT or not it is a wear item that just needs replacement. Luckily you can still get CS-M770s which is the latest XT 9 speed cassette if you want to keep it XT otherwise PG-970 or 950 will work.