I had the same problem, and it was particularly bad when the AC was running. It turned out to be a vacuum leak caused by a cracked oil filler tube. I had a look on another site and found out that oil filler tubes on cars '00 and older have had a Technical Service Bulletin issued on the part: the plastic is weak; the part has been updated. Sure enough, my old oil filler tube had a nice coating of oil and dirt on its underside. I pulled the old part from the car (PITA) and had a look. It broke in half on the way out. Being too impatient to wait for the part to come in, I took it to the local Advance and found a universal radiator hose that was the same length and that would bend to the shape. It cost a whopping $14.00 with screw type clamps. My car idles perfectly now, and the hose looks right at home on the motor.
