Weird... I had/have the same problem in my driver's side rear door! I was able to diagnose the problem when I took off the door panel. (I noticed the problem because the door would unlock about 5 seconds after the rest of them, and I could hear air escaping. The vacuum pump would also stay on longer to fix the loss of vacuum, or so I'm guessing).
Well I took off the door panel and could hear the air escaping not from the vacuum line, but a black rubbery peice that connects the vacuum line to the mechanism (actuator?). Well I noticed that when I pushed on this black thing, that the air could barely escape. So I shoved something (I think a fuse puller?) between the black peice and the inner of the door frame, thus pushing on the actuator (?). It solved my problem, and has stayed that way for about 4 months now. I expected only a temp fix (knock on wood, burl or zebrano of course!) and I seemed to have gotten a permanent one, or so we'll see.
Anyway, take off your panel and let us know what you come up with.

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)