Originally Posted by dwoloz
Err well I sometimes get confused about upshift versus downshift
I'm talking about bigger cog going to smaller cog
Going from small to large is no problem, it shifts smooth as butter. Going back down is where it gets hung up
That's an upshift.
This is often an indication of excess friction due to a dirty and/or corroded shift cable and cable housing. With STI shifters it's best to replace both shift cables and cable housings on a fairly regular basis. The short housing just before the rear derailleur is the most critical.
Also flushing out the shifters with WD40 may help.
Al