Most likely a vacuum leak somewhere. I had the same issue twice and fixing a leaking vacuum line fixed them both. Follow this link for a good how-to on troubleshooting and fixing a vacuum leak Mercedes Vacuum Troubleshooting

Glenn

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)