I have a problem with the rear derailleur - shifting to any sprocket works fine except for the second biggest one. If I shift into it (no matter in which direction), the chain starts trying to go into the largest sprocket, but won't stay there either and switches back and forth between the two. All other seven speeds work fine, so I think that the RD is aligned properly.

My system is 9-speed Sora (3500) STI shifters, Deore (M592) 9-speed RD, and an SLX (HG80) 11-32t 9-speed cassette, SRAM 951 chain. I know it's a mix of road and MTB components, but it's supposed to work, as far as I understand.

Thanks!

Have you had the usual suspects checked yet? Der hanger alignment and cable tension? Andy.

Maybe your cable is a smidge too tight so it's trying to pull it onto the largest sprocket. Try screwing your barrel adjuster in the tiniest fraction of a turn and see if that makes any difference. If you know how to check your low-limit screw, make sure that's okay while you're at it.

e: On second thought, don't touch your limit screw. If it's overpulling for the 2nd-largest it's probably doing the same thing for the largest, but the limit screw is doing a good job stopping it if it rides well on there.

Try turning the barrel adjuster about 1/8th of a turn clockwise

Given that it only happens on this sprocket, try to focus on what may be different. Step one, which is unlikely to find anything but may spare you useless work, is to use an improvised feeler gauge to make sure the sprocket spacing is identical to the others. I sometimes improvise gauges using the largest hex key that will slide between sprockets, then note the angle they can twist to.

Assumeing all the spacing is fine, it might be related to the "B" screw setting. If the upper pulley is too close to that sprocket, it can cause similar issues. The B-screw is especially suspect if it has the problem using one chainring, but not the other.

I wish I could offer more, but this kind of thing needs careful eyeball observation to find what may be a very subtle clue.