2.5 dt overheating when towing..... help please

hey people had a p38 4.6 hse for a couple of years and towed my 2 tonne caravan no problem. recently sold it and bought a 2.5dt. we went in the caravan this weekend and as soon as we got on the mototway it started to overheat, so pulled over checked water level, fan etc but no probs i then rang guy salmon who also couldnt tell me why this was happening. so we carried on with the heaters on full windows down the temp gauge would go up when doing about 60 mph but drop to 50 mph and would go to normal then come to a hill and no guts at all and over heat. with out caravan on temp stays normal even when booting it for long periods. does anyone know the answer or anything i can try thanks.

Viscous Fan coupling shot, or radiator sludged up or, water pump impeller blades breaking up.

All the above. Don't take any chances and replacethe lot including the reservoir cap.

cheers for that lads 9 months ago i had the radiator and water pump replaced i will try the viscous fan cheers

Or just loose belts (treched after 9 months use) after refit