I recently changed a leaking thermostat housing and thermostat on my 2l petrol 2000 mk2 mondeo. Now I find that at idle it is fine, I can leave it idling and the fan cuts in an out, and the temp stays about the quarter mark where is usually is... all is well. Driving in backstreets at about 50k's it is fine. However if I'm driving at 80k's or above for any more than a few minutes, it will overheat when I come to a stop. I have bled the air out of the system, and performed an engine leak down test, and a pressure test on the radiator, which showed no leaks to or from the cooling system. Going to flush the radiator tonight, any words of wisdom on other things to check while trying to nut this one out as I've never had a car that only overheats upon stopping after travelling at a high speed.
If you're confident that the coolant lines are properly bled and the fan is running as it should, overheating under high loads is usually indicative of insufficient coolant flow.
I would have the water pump checked out to make sure that it is still capable of delivering the required flow rate at higher loads.