Engine Cooling problem
1998 Other Vauxhall Models Four Wheel Drive Manual

i have a vauxhall frontera 2.5 tds runs fine no overheating unless on the motorway doing 2 or 3 junctions at 70mph then it overheats and the oil pressure drops but as soon as i get off the motoray it gose back to normal had headgasket checked and that was fine can anyone help please