I had the same problem with my C280 (same M104 motor).

One (or more) of the cylinders is mifiring bad enough, so that it shuts down, preventing damage to the catalytic converter (unburned fuel in CC will ruin it).

I had the spark plugs (Copper, NOT Plat.), plug wires, and coil ends (X3 of them.. 2/cylinder) per Arthur's suggestion.

Car has run like a dream since then! 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!)