what happens is the fast idle valve in the throttle body becomes stuck. this is a mechanical idle control valve that is controlled by a temperature sensitive spring heated by those pecky coolant lines going through your throttle body.

it's supposed to close when the engine is warm and your IACV takes over from there, in a nut shell.

well, when it sticks, it's a HUGE vacuum leak that the IACV cant compensate for. on the bottom of the throttle body where the coolant lines go, you can unbolt the assembly and either weld the throttle body closed of make a block-off plate.

9 times out of 10, this solves the issue