The shift pivot bolt is probably the culprit. Loosen the pivot bolt jam nut at the inside of the pivot 1 turn..........try turning the bolt CCW one turn. Then CW one turn. If the shifter moves with the bolt rotation, you have found the problem. The pivot bolt is threaded into the frame and has a jam nut locking the bolt in place. Remove the bolt and apply liberal amounts of grease to the shifter pivot bore n reinstall.........Good luck !