Detach the cable from the der's anchor bolt. Hold inner cable as close to the shifter as you can, maybe slide the casing off to get right up to the shifter cable port. While slightly tugging inner cable work the shifter through it's range. What happens? Does the cable move with each shifter position or not? If you start this hold the cable and shift close to the der then move the holding point up along the cable's route sometimes you'll find that the cable is snagging along the route at a specific point. Key to figuring these things out is to separate the various functions (der movement, cable movement and shifter function). Andy