I recently got an XT M761 rear derailleur that shimano has listed as 9 speed compatible and my LBS installed it on my bike with my 8 speed HyperGlide cassette and an 8 speed rapidfire shifter. My LBS had it listed in their system as 8/9 speed and said it would work with my setup. Now after about a hundred miles on it it's starting to shift without my input usually when it's somewhere in the middle of the cassette and more so when I'm pedalling hard. This is actually why I had my previous derailleur replaced - it was exhibiting the same symptoms and I was convinced that my Deore derailleur had worn out after approx 1500 miles. I checked my cassette and there is no play in that so I don't think that is the problem.
Is this happening simply because my new derailleur isn't compatible with an 8 speed setup?
You may have a bent derailleur hanger, excessive cable friction somewhere, or a simple cable adjustment issue. But it is definitely not the "8/9 speed" thing that's causing the problem. Regarding that, check posts #2 and #5 of this thread: Recommend a rear derailleur, please. needed urgently.
Rans Enduro Sport, Hase Kettweisel Tandem, Merin Bear Valley beater bike
Your derailleur isn't the problem.
I pretty much agree with well biked. I'd say that your derailleur was suffering from cable stretch but for the fact that you said it had done the same thing previously. Knowing that, the first thing that I would do would be to check the derailleur hanger alignment. That's a fairly common problem and the more rear cogs you have, the more critical derailleur hanger alignment becomes.
Proof that an XT "9 speed" rear mech works with an 8 speed cassette is available by clicking on the link in my signature to pics of the Max. I now have an XT 8s cassette before anyone mentions the temporary non-OCP one in the pics!
Cable stretch would be about right for 100 miles after a new cable is fitted. Rear mech alignment issue would have to be caused by something (crash, drop, being caught on something...) if it was fine before. In general, unless you have an off, any shifting problems that occur after a period of normal behaviour can be attributed to cable stretch or wear of components (cassette teeth, chain, etc.)
Thanks for the replies guys. I'm going to have to look into this more. This is on my commuting bike and I didn't crash or anything so I don't think it was damage related. My cassette has a few thousand miles on it so it may be worn out.
By any chance is it a full suspension mountain bike? From my experience, they can be especially vulnerable to ghost shifting. A good look at the derailleur cable, making sure it's as friction-free as possible, is important-