Derailler problem

DaveB said:

For some time now I've had a problem with the derailler on my road
bike (Tiagra). Around the middle gears it has a habit of jumping
between gears. I've replaced cassette and chain but discovered
mid-race yesterday that it is still happening. On the bike stand it
seems to be adjusted fine with no jumping, but under pressure, ie.
standing on the pedals going uphill, is when it happens again. Anyone
have any ideas?

My first port of call would be to replace the shift cable, but only
after I'd had a long hard look at the cable guide under the bottom
bracket (assuming your bike routes the cable this way) and replaced it
as well if it shows any wear.

A high quality Teflon coated cable helps.

Given that you have done all the other things, if it is not the cable
or the guide binding then start looking for less likely things like
cracked/broken frame tubes.

--

DaveB said:

For some time now I've had a problem with the derailler on my
road bike (Tiagra). Around the middle gears it has a habit of
jumping between gears. I've replaced cassette and chain but
discovered mid-race
yesterday that it is still happening. On the bike stand it
seems to be adjusted fine with no jumping, but under pressure,
ie. standing on the pedals going uphill, is when it happens
again. Anyone have any ideas?

Maybe this: http://sheldonbrown.com/autoshift.html

John