SLK230 rough idle when in reverse
 

My 1998 SLK230 just started having a rough idle when I put it into reverse. It seems to go away after the engine has warmed up or is put into drive or park. It currently has about 108,000 miles on it and has not ever given me any other problems. Does anyone have any idea of what could be causing the low idling and how it can be corrected?