Disco 4 (LR4) Gear box fault when towing

Hi I’m new to the forum,I’ve got a disco 4 tdv6 3.0.2010 model I’ve had it two years,no major problems with it,it’s been a good reliable family car,the only problem with it is when I tow our pretty big caravan, sometimes we get half hour into our journey and the revs start going up and down,then it puts itself in neutral and displays gear box fault on dash,I can turn the car off and start it again and it will be fine for another 20-30 minutes then the process repeats,any ideas anyone. Help!