Are you sure it's not normal? Both mine and my dad's (now totaled) C230 do something like it, with an occasional "dip" in idle speed when stopped in gear, like something is "kicking on" sometimes. Back when W202's were service loaners I noticed they did it too. The idle falls to 4-500rpm for a second and then comes right back. Never stalls, never has.
Lots of C-Class owners complain about it.
1998 C230 "Black Betty"