On my 98 lude after warming up to operating temp. and only in park the car idles up and down from 700 to 1200. goes away when it is in gear, but if the car is warm and I shut it off and then start it up it keep it in park than it is fine until I put it in gear and then go back to park and is surges. crazy ludes I guess.

 

DUDE, idle problems are all over the archives, dont limit your search to preludes, all hondas have the same idle issues, do a search for my name and idle.