I paid $ 125 for transmission fluid and filter change at a Mercedes dealer. This type of service I only have done at a a Mercedes dealer, due to the sensitivity of the transmission.
Also, the trans was obviously replaced under warranty, and obviously defective. But you now seem to have a somewhat new transmission! I'd be happy about that!

As for the kick-down switch, is the switch working properly? Is there a resistance point at the end of the petal? And then does it "click" when pushed further? If not, then your kickdown switch would likely be the culprit, I believe about a $28 part.

Good luck and let us know what's happening!

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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!)