Poor towing

Can anyone advise i have a 1993 disco 3.5 in general motoring no problems but when towing i have noticed that the metal shaft on the auto box gets hot and it drops down to about 30 mph on hills and really struggles temp wise it sits half way and climb up on hill then drops on the level where do i start any ideas

Doesn't sound too unusual. My auto RRC is broadly similar when fully laden & towing the boat. On the long hill on the A30 at Fraddon I usually finish up dropping to 2nd gear. Solo its not a problem.
How heavy is the thing you are towing?
Do you select a lower gear to help with the hill climb?
No real surprise if temp goes up slightly if engine is working hard.
If it runs fine at all other times I wouldn't worry about it.