Originally Posted by
shelbyfv
This is incorrect. Check that the inner cable isn't frayed or otherwise impeded in it's movement through the housing or in the shifter itself. The symptoms actually sound more like an incompatible rear derailleur but I assume you've ruled that out. Good luck!
The cables and housing are new so it can't be that. The derailleur and shifters both came from the same bike (to my knowledge) which must have been working at one time. The derailleur is Shimano Tiagra. I thought basically all Shimano indexed derailleurs and shifters, except Dura Ace, were compatible as long as you used the right cassette (according to Sheldon Brown). Seems to me not shifting evenly up and down with new cables and housing indicated the ratcheting mechanism inside the shifter is malfunctioning, because it's not pulling and releasing the cable evenly..