This year, my trusty commuter with 9 speed STI has developed shifting problems. Specifically, it sometimes shifts 2 cogs rather than one. Other times it shifts one gear and then pops over another. When this happens, there is no unusual drivetrain noise and I can easily get it in the gear I want by shifting again. When it slips, it can go up or down a gear.
I think it is adjusted properly -- when I put the bike on a stand, there's no noise, the alignment is good, and tightening or loosening the cable any significant amount messes up shifting. I thought the problem might be a sticky cable and replaced the cable (which was replaced most recently last year and was still in decent condition) but not the housings.
Could the problem be related to my RD? I have over 40K miles on both the shifter and the RD which I've replaced the pulleys on multiple times just in case there are wear or lifespan issues with these components.