I would check the compressor's clutch out... I had the same problem on my 85 300D, and it turned out that the clutch would only engage at higher RPM's...all it needed was an adjustment. My mechanic had some special tool he used...sorry I don't know what it's called. He explained it to me as wear had caused the clearance on the magnetic clutch to widen.

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles