overheat problem on a land rover discovery 3.9 V8i gas conversion?

any time I drive for a distance of more than 30 miles, the temperature starts to rise towards the red and sometimes into it. Have been told it is possibly an airlock in the water somewhere, but I'm not convinced. If i sit for ten minutes, the temp drops and stays normal for another 30 miles or so. have also been told to run the heating on hot for a while, as this helps.