Has it always been like this? Maybe a heavy car with a 6 cylinder just doesn't have the power? But, not sure if this works on your model year transmission, but, try this.

While stopped, in drive, with foot on brake, shift to the low gear position, then back into drive. This sets a one time first gear start. It works in my SL.
And the car launches much better.

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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