Diagnosing shifting problems
 

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.

Are you due for a cassette/chain change?
Did you check for lateral play in the RD?

Cassette and chain have been changed recently -- this is definitely a case of skipping cogs rather than on the chain itself. I have not checked for lateral play in the RD. If I find some, how do I go about getting rid of it?

40,000 miles on a one rear derailer? There may be enough lateral play to cause this problem. The only way to fix this is to replace the rear derailer. I've had a similar problem before from a worn out rear derailer.

Check for a bent derailleur hanger.

It's best to replace shifter cable housings when replacing cables. Actually the housings wear more than the cables. The short housing at the rear derailleur is often the culprit for shifting problems. It gets more dirt, water, salt, and corrosion.

You could also try hosing out the shifters with WD40, although this problem sounds like something else.

Al

Just fixed a friend's bike with the same amount of miles on his STI shifters. Spray the hell out of the shifters with WD of some other light lube while flipping the lever up and down the range (oh yeah remove the cable while doing this). Do it several times then, spray some Triflow or equivalent over the mechanism. Should give the levers extended life. You may be getting to the end of the useful life of your STI levers so start saving.