I'd have to check my FSM about your fast idle set-up on a 3.5 liter engine. The way the older 3.0's work like mine is this, there is a fast idle valve at the bottom of the throttle housing. Bolts to the bottom of the throttle actually. It's coolant temp controlled if I'm not mistaken. <img src="/forums/images/graemlins/notooth.gif" alt="" /> The valve when cold should be open to allow more air to bypass, which gives you a higher cold idle . Then, as the coolant heats up from the engine running, it slowly closes, and your idle drops down. The stepper motor, which is just above this valve takes over idle duties at all other times. Make sure your engine coolant temp sensor is working as well. For a 3.0 liter 12 valve engine, it plays a huge role in keeping the stepper motor control by the engine ecu accurate. Mitsu's number 1 check when using the FSM for idle instability is to check the NTC or coolant temp sensor. Usually if it's this bad you'll set an intermittent Check Engine light as it moves in and out of specs. For a 3.0 liter, you get fault code 21, probably a different code on the 3.5 liter engine like you have. I'll check the FSM tonight if someone else doesn't have more/better 3.5 liter info for you.
You may only need a gentle cleaning of the stepper motor and the section of the throttle that it sits in.