I have a Specialize Roubaix that's about 10 years old with the Shimano 105 component set. I've started to develop a problem in the rear derailer where when I shift to a higher gear/smaller cog, the shifter "jumps" multiple gears. Happens whether I'm on the middle or big chainring in front. And yes, I know it's jumping multiple gears because it takes multiple shifts to get back into the gear I started from.
This seems to be the opposite of the stickiness most people complain about. :) Any ideas on how to address it, short of replacing the shifter? I've already replaced the cables, adjusted shifting, checked the chain for wear, etc.