Could be a sticking valve, but you might want to take this to a dealer or an independent mechanic that specializes in Mercedes cars.
Where are you located? I can point out some good shops in Southern California.
There are a lot of shops mentioned in the Good Shops forum...
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".