Is this a totally random occurence or was something recently changed?
If its random, I wouldn't doubt that a vacuum leak has developed. Check all the hoses behind and underneath the upper intake, and on the driver's side vacuum tree. Spray some carb cleaner around the hoses, if the idle changes when you spary at a particular spot, you've found your leak.
If all of this fails, do a search on here for setting the idle. There are plenty of good write-ups on how to do it properly.
After what he said, look at your IAC valve next. If you do determine that it's the culprit, replace it with a borg-warner part only. Others will not work.
When my IAC valve busted, if I took the car all the way through first gear to 5k, then let off, the idle would return to normal until I shut it off and restarted the car...shrug:
Scott
I would check for vacuum leaks, it's a common problem with our hoses being 12 years old. They get brittle and leak. I have also had trouble with my throttle fluttering and I cleaned the IAC motor and the problem went away.