Pulled the EGR valve today. There was carbon build up around the seat, plunger, and seat chamber. The valve could not close becuase it was so fouled up with build-up. Cleaned it all up with dentist's tools. The engine hasn't idled this nicely in a long time. I guess it has been getting worse over time and I simply didn't notice it.
All fixed - thanks for the advice!

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Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport