I have 8sp campy chorus components(older ergo). When I'm in my 21 tooth gear and the small chainring(42) I can feel a sort of grinding or ratcheting in the pedals. There isn't any noise but I can definately feel it in the pedals. It disappears when I shift to the 19. I brought it to the local bike shop and he said my chain was stretched out so I replaced it and it help but it came back again. Could it be that I need to replace my 21 tooth cog, chainring and chain?