Just recently I notice twiced on my last ride across town something that has never happened before. I was in slow traffic going along in 1st gear for awhile (probably a 1/3 of a mile) at about 2600rpms. As I came to a stop my engine idle stayed higher than the normal lug lug. I rolled my thottle thinking it had stuck but it was at bottom out of rotation. But it kept idling fast. It did this on two different stops and then went away. Any ideas what would cause this ?