Ok, so your kick-down is working properly.......
the problem lies elsewhere. JSmith has a somewhat similar, but more sever problem (I believe). Maybe he can give you some clues on where to go from here.

How else is the operation of the transmission? If you have it floored into kickdown, will it redline before shifting into the next gear?
Some adjustments probably need to be made.
GOOD LUCK.

__________________
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!)