Same problem on my W210 was caused by low charge - this is VERY common on the W210 and the duovalve issue is not as prevalent. The best thing is that it's a CHEAP fix!

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280