auto gearbox problem

I apologise in advance if I've put this in the wrong place?
I have a Land Rover Discovery, 1999, V8, Auto. Whilst going uphill or on level ground, my discovery will not change up from 3rd to 4th unless I de-accelerate, it will then change to forth but when I try to accelerate again it will kick back down to 3rd and the cycle repeats itself. If I keep my foot hard on the pedal (still in 3rd) the engine rev's are approx' 4500rpm but the speed seems to just level out and in some cases slows but it won't change up?
When travelling slightly downhill it appears to be ok as long as I don't try to accelerate fast? I have replaced the ECU unit (under the passenger seat) but the symtoms are still the same? It appears to be OK in the lower gears. Can anybody offer me an explanation and remedy?

try changing the oil in the box, run it for a week, then change it again (you can only drain about half at one time because the rest stays in the torque convertor).

Hi, thanks for the advice, I will try that.
Would it be wise for me to change the filter at the first or second change also?

if there is one, yes - at the second change.