Rough idle on 2005 9-5 Arc sedan
 

We just purchased a 2005 9-5 Arc sedan with 72000 miles.


It runs O.K., but the idle is a bit rough. The RPM will increase for a second, decrease for a second, and then steady out. Four seconds later the RPM will increase for a second, decrease for a second, and then steady out. Over and over. It seems worse when it first starts.


No error codes have (yet) been displayed on the alphanumeric display.
Any ideas on what to check first?