Originally Posted by IANS2RST

have you bled the system properly? sounds like there is not enough water in there. when stationary it will get to temp as there will be enough water in the header tank. when running at speed the first thing to suffer is the heaters if there is not enough water in the system as it flows round. fill the cooling system until water flows out of the of the bolt hole on top of the thermostat housing & then leave running with the header tank cap off for about 10 mins occaionally holding the engine at higher revs.