I've had no end of grief with Natalie's Zetec s

I've looked on the Zetec S site for this problem and it seems the cars are prone to overheating and they can't fix it.

Standing still its fine, but it overheats if the heater control is not on hot when driving if the heater is on cold at 50+mph temp starts rising.

Tested the H/G, changed the water pump, thermostat and rad it still does it

I'm going to change the heater valve which I've changed before and the engine fan as when it starts getting hot if you turn the internal fans on the temp goes back down.

Fucking hate that car