Well here is a strange happening.Car Heater

[Number 6]

The heater in the car (75 Diesel), has all of a sudden started to play silly luggers,, From cold with the temp set at 20 both sides the hot air starts to increase as the engine temp rises. All good, but after 20/30 miles at a sustained speed cold air starts to blow thru the vents, turn the temp up and you get warm air. There are no sign of over heating. The pipes to the heater matrix are both hot when the cold air starts to come thru the vents

Any one have any suggestions??