2002 Disco TD5 - When cold the gear lever operates as it should but after about 50 miles of driving it becomes very sticky when moving across neutral and is especially noticeable when changing down from 5th - it doesn't automatically fall in line with 3rd/4th, the same hapens when moving off from rest and changing from 2nd to 3rd. The gearbox oil level is ok - any help would be much appreciated. 