Sorry if the previous post seemed rude. Just needed the info... :^)
The auto climate stuff is a mix of vacuum to activate the various things and a control module (brain) to tell the vacuum what to do. I would guess that it's the control module. The vacuum would not be intermittent or would disable particular functions all the time (the lower vents don't open on my car, for example... :^( ). The fact that the defrost works seems confirm it: defrost doesn't think, it blows air at the maximum setting as hot as possible.
I remeber reading about control module boards that would crack and create intermittent problems with connections coming and going. Might be what yuou have.
I guess the folks here can give you more info on all this.
1982 300D - <A HREF="http://members.xoom.com/nicodore/