My car, 83 300D Turbo, 319K miles seems to forget the heat setting after about driving for about 30 mins. I get heat blowing thru the 2 side vents. After about 30 mins, it then starts blowing luke warm air thru all 4 front vents.

I have to turn the dail on the CCU up (more heat) for the heat to come on again. Heat once again comes from the 2 side vents.

Is this normal for the heat to go luke warm or is some unit in the car that needs attention ?