The problem mine had was with one of the yellow vaccumn motors near the master cylinder (opposite side of the car for you?) It cost me $7.50 US and has fully solved the problem. By the way, the car generates maximum vaccumn at idle. As for other suggestions, try this: remove the wood center panel which houses the climate dials. There are two screws right above the radio on the bottom of the panel. Once these are removed just carefully pry out ot panel. Then remove the climate control unit (There are a bunch of small silver screws set in the black plastic for this) Once the CC unit is out, you should see a good-sized vac. motor with a connecting rod on one end and a hose on the other end. When you take the CC out, don't unplug it. Start the car, and put the CC on a cool setting. If it blows heat (do this when the car is still warm), try grabbing the rod on the vac motor and moving it one way or the other. See if this makes a difference. Alternately, by the nature of your problem, could it be the overhead sensor located near the sunroof switch? Worst case scenario is that it is the CC Unit itself. Good luck.

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro