my disco will run all day at 60 mph if i go up to 70 mph on a long hill she starts to over heat. i have got 235 x 85 x r16 tyres on her so they are not overly big. can anyone give me some advice what to change first

can anyone help solve a problem with a 95 disco its ok at 60 mph all day go up to 70 mph on a up hill it starts to over heat the tyres are only 235 x 85 x r16 so not over big has anyone had the same problem