Hello and help!!

Hi there, my name is Lee and I am new to Land Rovers. I just sold my Toyota Hilux as it was a bit more than I needed and it only had two seats so if we were taking my son caravanning, my wife would have to follow us in her little car Anyway, I have now bought a cheap, old Freelander td4 mk1. Which I thought would be perfect for our purposes, towing a caravan on/off sites, commuting and the odd towing job for work. I was pleasantly surprised when I started driving it and was remarking on how well it pulled up hills to my son. Comparing it to the old straight 4 diesel in the Hilux, it was a dream to drive. Then, the very next hill, I put my foot down to overtake and the revs dropped, the speed dropped from 70mph to under 50 and foot to the floor it would do no more until I got onto level ground and then a decline. I decided then to put my foot down and see what it could do when it wasn’t going up an incline and it wouldn’t do more than 80mph flat out It was cheap and it is old but i’d still like it to function properly. I looked at a link on this site from 2013 where someone had a very similar problem but the link ended without saying whether the problem was ever resolved! Has anyone please got any advice, or experience with this? Thanks for having me