different overheating prob

[nidul]

Right hear goes guys, my dillema is that i have changed my fan coupling and thermostat and filled up with new coolant. But my car still gets very hot when driving at high speeds of 70-80 mph but cools down at lower speeds. when im doing high speeds the temp gauge goes up to three quaters or just over then i put the heater on full then it cools down. i was told it could be the water pump, does this sound right and if so how easy is it to change this pump. thanx for any help in advance

[TOURINGDADDY]

does sound like a foobard pump mate

[325icab]

iv got a pump, urz 4 a tenner