Ssangyong Musso BTR 74LE transmission (1999)

Reverse gear is very very slow to engage when the GB is cold drive 100 meters about 30 ft and it will engage, but leave it standing it soon goes back to being difficult to engage. This has been a problem for seven years. I very recently changed the oil and filter, the oil pan was clean with no swarf on the magnet. All gears shift smoothly with no slip