ZF Disco, Auto Gearbox problem

Hi, CAN any one please please help!
I've just recently bought a 1996 Land Rover Disco,ES 300TDi Auto
and as i have a disability with my left knee after a very serious accident i really love the driving position of the discovery with the auto box.
In getting to the nub of the problem, on the way home back to N.Ireland, and into my journey about 2 hours it was going fine and was running excellent, when suddently the gearbox changed down into third gear from fourth, the problem is it would not change back up into fourth again without slowing right down to 10 Mph, then it went fine until i reached the speed of approx, 50 Mph in which it would change down again, and would not come back up again unless i stopped. The other gears 1st and 2nd are perfect as is third, and fourth also, when it is engaged, until i either get above 50Mph or the road conditions call for a change down in which case all the above happens.

What is also confusing is that, when i got off the boat in Dublin and the Disco had been sitting for a couple of hours, it then drove all the way back up to Belfast approx 200 miles in the pouring rain without any problems what so ever. The next morning i took the Disco, out on the motorway and this time at about 70Mph the same as mentioned before happened.

I would really appreaciate your assistance as to what is most likely to be the problem before i have the Box stripped down or replaced. Many Thanks. Gordon.

hi gordon just read your post and got email i will have a read up on rave and see what components are used in conjuntion with gearbox see if i can get a better understanding and see what electrics are involved if any cheers tiga. hi gordon had a good read and all i can see is that the gearbox is just operated by various oil pressures operating valves,in the fault finding section it says possible cause is 3rd to 4th upshift valve sticking or 3rd to 4th down shift valve or 4th gear clutch,so at this time cant see any other choice but getting gearbox over hauled sorry,tiga

Hi,all, just a short note to say that i've now got my Disco's, Auto Gearbox sorted out thanks to David Ascroft of Ascroft Transmissions.
I asked Ascroft Trans, if they had any possible remeady's regarding the box before i had it pulled out, David Ascroft suggested adjusting back the
kick-down cable a couple of turns, and i also had the oil and filter changed as well.
I can now say thank God the gearbox is now as sweet as a nut.

Hope this may help any one out there in a similar situation.

A big thanks to tigapiglet too, for all his advice. Would recommend Ashcroft Transmisions Highly. Redfrogg.