when you say it overheats does it actually boil over or just go up on the guage,,i had this very same issue with my old td i replaced rad,replaced water pump,replaced temp guage,replaced thermostat and still the same problem mine used to go to the end of the white section on the guage and i would panic and pull over and wait for it to cool down,then a friend of mine suggested lettin it run past the white setion but not quite into the red,when the needle got just past the white the thermostat would open and the engine would cool to the point of the needle sitting half way and then runnin up after a while,it could be that the point on the guage is the point where the thermostat reaches its operating temp,but if it actually boils over could be a crack in the head