Sometimes when I start the car on a cold start up, its idle raises to about 12k then DROPS down to about 1-200. it only does it randomly. once I get it into drive and start moving, it drives great! but when I come to a dead stop it does it again.
When I try to show people it doesn't do it though. I've drained air from the cooling system, did my best to check for a vacuum leak, replaced most gaskets in the car (think i missed the camshaft seal, and crankshaft seal) and it doesn't seam to help.
only check engine light code is for my catylitic converter but I'm gettin a new exhaust soon.
any help would be appreciated.

 

IACV

 

Clean your IACV.. I had an issue similar to this, and it resolved the issue.