On the way to work this morning after cruising at about 80MPH for 20 miles, I got off at my exit and the car would not Idel down. Stayed at about 1500-1800 rpm in drive. I put the car in Nuetral and the RPM'S went to about 2500 and then went back to normal after tapping the throttle a couple of times. Did this two or three more times on the remainder of the drive. No CEL, no messages. Any suggestions??