JP. Am having similar problem with my '90 300E. Seems to be vacuum related. Have been told by mechanic that the flap-actuating vac solenoids in the dash are difficult to access. He is an independent M-B tech with lots of experience. He has a "vacuum system analysis" that he claims will pinpoint vac element problems and costs about $79. This, of course, is not including any repairs or parts. Anyone heard of this procedure? Am considering it, in the meantime will watch this for additional posts and add any info I can.