I am having similar problems with my '92 300SD (W140). The A/C runs fine for a while on the first run of the day and then the rider's side vent starts to produce hot air. After a while the rider's center vent does the same and then the driver's side vent. It happens most often after a stop. Seems to run pretty well on a long trip. You can fix it up by playing with the defroster button most times.

I have looked around and found a vacuum line labeled "vent line to passenger compartment." However, it is not plugged into anything. I have only the diagrams for the engine compartment, but this line comes up from behind the passenger's headlight and is carefully threaded between the computer compartment and the cabin air intake compartment (with filter and fan) and it just ends. It is not plugged or plugged into anything, nor can I find any other line in that area (or anywhere else) into which to plug it. I can't imagine that they would put an access to atmospheric pressure at the end of such a line. Everything else in the climate control works OK. I'm wondering if this line goes somewhere or is just supposed to provide venting to the outside.