Discovery 1 300tdi auto getting sluggish when hot

Hi there, my discovery 300tdi auto runs fine then once I start abit of off-roading it gets more and more sluggish and won’t rev or pull its self outta the mud seems like it’s getting hot or automatic gearbox getting to hot fluid is fine and looks new still, any one had problem like this? Thank

If it wont rev then it isnt the gearbox, start with filters and check of boost hoses i reckon

It will drive down motorway 70mph no problem all day as soon as it goes near abit of mud I bogs down and will not pull it’s self out or Rev up