Maybe you're looking at the wrong area for your problem. You said the gearbox oil is fresh, so presumably any burning of the clutches/bands would have been recognised at the oil change..................
If the engine is not in a good state of tune, then you will be applying more throttle to compensate for the lack of power. Using more throttle "tells" the autobox to use different change up/kickdown points.
Possible problem areas?
Engine tune, assuming it's diesel, worn injectors/pump/fuel filters/air filters choked/tight valve clearances/low cylinder compression...............etc.
What I'm getting at is, make sure the engine's in tip top shape and the 'box will follow on - it's very throttle/load dependant.
Oh, also make sure your brakes aren't binding!