TD5 automatic gearbox

I have just bought a 2002 Disco with an automatic gearbox. Two things I'd appreciate some advice on. First, often when I start the car, a clunk is heard from the gearbox - is this normal? Second, on occasions the gears have got stuck and won't change up - this has happened mostly in first but sometimes in third. The only way to change gear is to slip into neutral and then back into drive. Does anyone know what causes this?

a few things to check first the selector cable if there is any slack and also the selector switch located on the left side of the box see that your able to start in neutral and red lights on gear lever match what gear your in .last but most important fluid level and condition in both main transmission and transfer box . ive had many dealings with the auto td5 ive had 2 auto discos and they both had there little problems .

Clunk on a gear box, Hummmm is this when engaging from park to drive on flat and level ground or on an incline, cos if you get a clunk on a incline, ie hill this is about normal for any auto gear box, and ive had vauxhalls, fords and rovers that have all done this.
If you get a clunk on flat level surface then theres some issue there...maybe selector or gearbox ecu

As for jumping gears, from 1st to 3rd, is this when youve just jumped in the car from cold and gone up the road....... again its sometimes deamed at normal behaviour of a auto box... Ive found this an issue with other vehicles but it normaly sorts its self out after afew miles.