I normally commute on my CX bike, which uses drop bars and Tiagra STI brake / shifter units. Last December the gear shifter on the left hand unit kind of imploded – it just jammed and wouldn't move. It was replaced under warranty in January (bloody christmas break cost me three week's commuting!), and has been fine since then, even after I broke the window in a crash back in April. Now, however it's started playing up again.

It's reluctant to move up from inner to outer ring (I run a 34.50 double). It will move eventually, but then won't drop back down again. If I release the cable though, it works fine – it only seems to have problem when it's under load, remove the load and it pops up and down with no problems at all

any ideas, anyone?

PS the good thing about having more than one bike is that there's always a spare to fall back on when your main bike is "hors combat" 😉

Cold weather related – could there be moisture in there which is freezing.
A session with a hair drier and then good dousing of GT85?

It's possible, but it's been indoors for the past 24 hours and it's still happening – the bike room at work is heated (to keep clothes warm) and the garage at home, while not heated, is definitely above freezing temps. It ha been in the garage for 3 days prior to it starting on Monday too

Just as a bump, the problems is a little piece of plastic in the mechanism which breaks off. It's a known issue and has been sorted under warranty. Winner.