so it is idling at 500 + or - or it is fluctuating 500 up and down? If it is fluctuating it could be a number of things...I know its tough to check for but see if you have a vacuum leak, Ive also heard of it being things such as a dirty IACV but in those cases it tended to stay more consistent in that it was constantly staying too high. Could be a vacuum leak and the IACV is compensating by raising it, then the leak lowers it, and so on.... or another thing to check out is the fast idle thermo valve which is located on the left side of the throttle body on the bottom kind of, it has phillip screws and is a beyotch to get to but it could be the problem if its dirty.... Also, try and pinpoint an exact occurance of this, does it only happen when you use power such as headlights, a/c , fan switching on? or is it all the time?