Check the oil in the gearbox immediately, tihs could be the gearbox stiffening up when getting up to temperature and could potentially be a seized gearbox.
Make sure the correct type/grade of oil has been used.
If the cable is still all the time and it is a fairly new cable, look carefully at the cable run. If the cable is cable tied excessively or with too many bends this can make the cable stiff.
Try dripping WD40 drop the cable from the control side.
Have a friend move the gear in and out and watch the movement of the gear lever on the gearbox.
Hope this helps