Quote Originally Posted by aicabsolut View Post
Sounds like it's a rear derailleur issue. It's common for it to seem ok on the stand but to start acting up under load. You probably just need a little turn of the barrel adjuster. Are there any delays in shifting one way or the other (does it keep skipping instead of shifting)? If that doesn't work, check that the B screw is set right and that the derailleur hangar isn't bent.

http://www.parktool.com/repair/readhowto.asp?id=64

(has pictures)
Sorry to butt in but I just wanted to say 'Thank you!!' for pointing this out. Why is it so difficult to convince mechanics in bike shops that just because it worked fine on the stand, the problem may not have been solved when you factor in the stresses of real life road riding? And why is it so difficult to convince them to just ride the wretched thing around a bit so that they can see for themselves? It's really helpful to be able to dip into the knowledge and experience here. And yes, I have changed bike shops until I've found one that will listen and am about to do a maintenance course too.