I have a 1991 Sigma/Diamante/Verada DOHC (368,000 kms). A year or three ago the climate control system started pumping hot air into the interior once in a while, irrespective of the temperature setting and whether the air conditioner was on or off.
This gradually happened more and more often until it was almost every day. The heat could only be stopped from flowing in by turning recirculation on and the blower off. After having the operating unit replaced by a second hand one the problem seemed to be solved. However, after a few weeks it started all over again. It even got worse, for after some time the blower sometimes kept blowing even when I turned it off and spells of hot air alternated with spells of uncontrollable cold air.
So I put my old unit back in. At the moment I get hot air almost every trip after a few kilometres. Sometimes it stops later on during the trip and very rarely I get cold air instead of hot.
I am sure it is not the interior temperature sensor, for when I turn the temperature wheel to completely cold, the blower starts pumping air inside at full power, unaware that it is hot, not cold.

My obvious question: does anybody know how to solve this problem. It seriously diminishes the pleasure I still get from this great car.

the climate control device and thermost

needs to be replaced/adjusted and looked

over by an automotive airconditioning specialist