I’ve been going through this for a few months (old hardtail frame, 9 speed). First I changed the cable – no good. Then swapped wobbly old 2008 mech for one from the spares box – still bad. Drivetrain had over 3000km on it, so I put on the new cassette and chainrings I had in the box, which had been waiting for spring – no good.
Must be the shifter, right? It is/was an XT unit bought 18 months beforehand. Absolutely blasted it with WD40 and lube. Seemed to help at first but shifting went wonky again. Replaced it with a new Deore unit – STILL not right! Useable, but just not right.
Bought a hanger alignment tool last week, bent hanger back into place. OK in the garage, taking it out Sunday/Monday for a test…