most likely a clogged IAC valve...its the box on the very top of the TB...the IAC controls air flow around the throttle plate (it gives electronic idle control for a non drive-by-wire throttle system, as in...the computer can control the IAC, but not the physical throttle plate)...
The IAC is susceptable to gunking up from a number of different causes...excessive blow by due to mechanical engine damage, or as simple as a stuck open pcv valve...a clogged EGR valve can also introduce a lot of carbon build up into the TB, and cause the IAC to stick as well...when it gets clogged up, it sometimes does its job...and sometimes doesn't...it seems yours is sticking open after the engine and everything cools down, but after warm coolant flows through the TB area, it starts working again...
test your IAC by resetting your ecu...just unplug it for a few seconds, leave the door open to let the dome light drain any residual current...and reconnect it...This will reset all of the 'learned' IAC steps the ecu recorded...For 50 miles or so, the engine should idle very low after fully heated...if you are still having a very high idle, its likely the IAC is either clogged or malfunctioning, as it should be responding to ecu control at that point (resetting the ecu will remove the possibility of a software bug)...some have gotten away with removing it and cleaning it...some had to replace it...sucks, as its not very cheap ($200+ last i looked)...but good luck man...
