"William Sommerwerck" wrote in message
...
Sounds like the thermostatic radiator valve that opens as the engine warms
up is jammed shut. When the car is running down the road, there's enough
air
flow to keep the engine cool.

Or, the valve's "opening" temperature has become too high, and it won't
open
up until the idling engine gets really hot.


That was my first thought as well, but modern 'stats are usually designed to
fail 'safe' i.e. open. How about an air lock? Some engines are very fussy
about getting the head full of water. I used to own a 5 cylinder Passat, and
you had to jack one side of the vehicle up really high to get all of the air
out, otherwise, it did just the sort of crap that you are describing ...

Arfa