Overheating

ok my truck keeps overheating i changed the headgasket and had the head checked and resurficed the truck does not get hot setting idle only when i drive it down the road (the hand does not move at all till i start to drive) it can start getting hot and i stop and it stops were it is at and stays there till i start driving again what could it be the water pump is pumping and im getting water through the Rad could it just not be enough water