as asked before... is the air con fan kicking in???? the reason i ask is because i had this on a saff years ago, identical symptoms, it is cold as you start up and pull away because the engine and rads are cold. when the engine warms up the heat bleeds onto the aircon rad and the air becomes warmer, the fan should cool it down if it works, mine however was f**Ked and didnt cool the air back down. i replaced the fan and hey presto it was a lot cooler. HTH