yo,
i just had the same exact problem, its the expansion valve. i replaced the compressor, which helped a little but the problem wasnt solved until i replaced the expansion valve. it regulates the pressure in the system. good luck

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1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350