Originally Posted by RobbieTunes

jiangshi's question was my first impression, that you were using brake cable in an STI shifing application. Mainly because 1) I've done that (still worked just fine) and 2) I've not seen those kind of ferrules on STI cables, just the black ones that are fairly tight.

Lacking that, I've got no clue, housing-wise. I've used the exact same Jagwire Road Pro/Racer kit on dozens of bikes, with no issues whatsoever. In fact, the time I used brake housing on 8-sp STI, it was when I went to a bike shop and that's what they gave me. I've seen bike shops do that more than once, actually, more than twice.

I have seen, over and over and over, the shifter cable able to be inserted in the 8-sp STI (especially the 8-sp STI) without the shifter being "zeroed out." Then, the problem you describe is encountered within a ride or two. That is my guess on this. At times, it is very difficult to fix, mainly because the cable does not want to come out. I've ended up cutting off the extra cable, and just monkeying with the STI until I could get the cable end out.

This is common because the STI shifter will shift on you, just by handling it, bumping it, etc. While you're installing cable, you can "zero it out," and then bump it while installing the cable, and end up with the same problem you describe. I've seen it too many times, caused it myself often enough, to predict with 75% certainty that this is the problem. Pull the housing, pull the cable, insert a new cable, and every 30 seconds while inserting new cable, double-check to make sure it's "zeroed out."

Good luck, feel free to PM me. You deserve a bike that works, and I've seen this too many times to figure it's not worth a try.