The very first thing you should be doing is checking the oil level in the autobox. To do this easily you will need a trusty assistant and you will need to get under the car to access the oil level/fill plug. I would advise you to get a couple of litrs of Dexron III atf oil in stock beforehand. You'll need a T50Tork bit or an allen key to undo the level/fill plug.
If you are not familiar with the process, here it is:
1. Start with vehicle parked level, handbrake on and engine cold;
2. Start the engine, let it idle, then using the gear selector, go through all selector positions, dwelling 2-3secs in each one to take up drive before moving on - then back to N. Then with the smaller transfer box lever select neutral;
3. Leave the engine running
4. You now get under the car and locate the level fill plug on the front face of the autobox - you'll find this plug just above two metal oilpipes - undo the plug, carefully remove it and see if any oil flows out. If a small amount flows, the level is correct, if a large amount flows it is over full and needs reducing, if no oil flows it needs topping up until it does flow. Replace the level/fill plug when the correct oillevel has been established.
Then check the functions of the gearbox.
Good luck
Dave
Ps If the 'box oil hasn't been changed for a while it might be a good idea to empty it out and refill. You will need 5 ltrs of Dexron III to do this.