Problem with air conditioner on 300 ce

[antonino]

At times the air conditioner on my wife's 1991 300 CE will not work when the fan button is pushed. It will go like this for a while and then, without doing anything, the fan will come on and cold air comes from all the vents. Then, out of the blue, the cold air stops and it blows hot air. This happens erratically and without touching the controls. Can you please tell me what the problem may be?

Thanks a million,
Antonino