Strange Gearbox Behaviour 4.8is !!!

[Neil48is]

Right,i've managed to get a few miles on the car since i finished it. One thing i have noticed is that when the gearbox is cold it tends to hold onto 1st gear till approx 3-3.5k Revs and when the box is warm it tends to change up too early. I find the car "bogging" down at low speeds in sharp bends,junctions etc. It never seems to be in the right gear. If i use the car in manual all the gears select just fine. Could this just be down to needing new fluid and filter in the box or is it potentially more serious??!!!!