1991 gearbox fualt

[scooby]

My 1991 r/r runs sfine but first thing it takes time in changing gear. I have to drive it for about 5 min stack in first gear once the heater needle hits the middle
and the car is nice and warm it will then change into all gears, Once this happens i have no faults with the car at all.
It has had a new gearbox but could it be the gear shifter control on the back of the box
I have found out as well today it will only change gear once i get to or over 40 mph not sure if this happens as the car is warm by then r not will try taking it out and get to that speed before it works ok. But as i say once it starts to work it will do all day. can not really get my head round it at the mo.

[Range Rover Blues]

Assuminmg an auto box, then as always start with a change of oil and possibly the filter.  Check the oil coolers are connected properly, one is in the rad and one ahead of it, if too cool and auto box won't work properly.