When everything is working right before, were the shifters hard to operate? I don't have much experience with STI's except for using 105 5800 for over a year now. It shifts very easily with little force. Couple things happened recently that made a notable change in the amount of force needed to shift. One was an out front mount for a gps that I put on the bars and since the cable came out from beneath the bar tape right next to it, a sharper bend in the cable caused noticable extra force needed to push the shift lever. Took the mount off and problem resolved. Also, on this old 90's bike frame I just ran the bare shifter cables directly on and around the BB shell as was common back then. However over the year, it cut down into the bb shell and started getting pinched which also caused lots of force to be needed on the shift lever. Added a plastic BB shell cable guide down there and all is again very easy shifting.
I can imagine that if left uncorrected, that this excess force would cause the shifter cables to wear, fray and break up inside the shifter. As it was, I found my issue in time. Even where the cables were cutting into the BB shell, the cables had no noticeable wear or damage even though the rear DR cable had worn a trench deeper than the diameter of the cable.
So if your shifting is anything but easy in the amount of force needed, there might be something else others working on the bike are missing.
Last edited by Iride01; 07-11-18 at 10:36 AM.