You can sometimes bring back a sticking cable by using PB Blaster and AT fluid dripped down the cable over a period of time.

Removing the cable and holding the end up with string or a cable tie lets gravity be your friend.

Grasping the cable with a pair of pliers about 1/4" from the end of the sheath allows you to force the cable back in a little at a time.

If it doesn't free up and move to a fully retracted position with the spring tension that is supplied at the other end it needs to be replaced to gain the original function.

However, if it will not move freely you can experiment with different positions of the cable and find one that will give you acceptable shift points.

If you err on the side of having it shift sooner rather than later you can always shift the car manually to increase the acceleration rate.

Randy