Originally Posted by
Freerojo
It sounds to me like your frame is flexing (under load). Same thing was happening on my wifes mountain bike. Go to an auto parts store and get Dielectric grease (couple of dollars). You may want to pull the cables out of the guide that is running under your bottom bracket, clean it and then put the grease in its tracks (the grease is made so as not to dissolve plastic) and re install your cables.
Good solution, but the cause is wrong. Cable friction, either in the housings, or at the BB creates trim problems and can lead to ghost shifting to the spring side (smaller sprockets for Campy and most RDs). but if frame flex were (is) a factor, greasing the BB guide wouldn't address it.