Auto Parts: 1999 Mitsubishi Eclipse Spyder, mitsubishi eclipse spyder, transmission oil change

Question
I received a reply from Anthony Falcone and researched the part he said I needed. MY PROBLEM:  I have 1999 Mitsubishi Eclipse Spyder 2.4L non-turbo, that has a problem with idle after it is warmed-up (It surges from between 1200 RPM's to about 650RPM's.  On occassion it will die.  This is an on and off problem-sometimes it does it, some times it doesn't).  Mr. Falcone said I probably needed a "Idle Air Controler" or IAC.  Mitsubishi says the closest thing they have to that description is a "Fast Idle Air Valve" ($300.00).  I have no problem with the "Fast" idle, but rather a surge when the idle is low and after it is warm.
Do you have any other idea's for me to research and try.  I think Mitsubishi just wants $300.00 plus $7.00 for the "O" Ring. This Eclipse is perfectly maintained.  Always has been.  Has New Timing Belt etc. Oil Change every 3-5k miles.  Transmission Oil Change Q 10k miles

Answer

IAC     Idle air control motor is what GM and Ford call the thing, but all EFI engines have one.  Even my old '76 Jaguar has a little valve in the manifolds that does the exact same thing.  What it does is allow air to bypass the throttle plate, in varying amounts which are controlled be the computer, to control the idle.  If the idle gets a little low, it allows more air through, which raises the idle.  If the idle gets high, it chokes off some air and the idle drops.  If you are familiar with adjusting the idle mixture on a carburetor (which nowadays you are likely not), it is the same thing.  Mitsubishi probably calls it the fast idle air valve, the price is about right.  RockAuto sells the thing, under "idle air control," for about $270.  Your descriptions of the symptoms would lead me to believe that this is the most likely culprit, especially given the age of the car.  It could be something else, however.  If there is a major air leak in the intake tube, even upstream of the throttle body, it could cause symptoms like this.  So, give that a good looking over.  You could get a pro with a proper real-time scanner to interrogate the computer for the function of the idle motor.  A good mechanic will have a scanner that can command the motor to move over its full range of motion.  If it fails to do so, it should be replaced.  What happens is they get gunked up inside, then the little electric motor struggles to overcome the resistance of the gunk and burns out.  I'll send a picture of the piece I'm talking about.