OakLeaf took the words out of my mouth. Make sure the cable isn't slipping through it's anchor. The cable shouldn't be changing every 100 miles. Also check every place where the cable enters and exits the housing, so see that the housing is intact, without wire stands poking from it, and the ferrules are not crushing or splitting.

Is this an MTB? I was told that the cable crossed routing works well on MTBs but isn't as recommended on road bikes. Make sure that the cable doesn't contact the frame anywhere because of the crossed routing.