First thing is to check there's no bends in the cable housing, then spray out the shifter w WD-40. If none of that works then a 21 speed grip shifter set and cables are pretty inexpensive. How is the headshok feeling?
__________________
Genesis 49:16-17
"Well, well!" said Holmes, impatiently. "A good cyclist does not need a high road. The moor is intersected with paths and the moon is at the full."