1999 Mitsubishi 3000 gt keeps reving up at idle and going down the road. What could it be?

I have had a simular issue. It first wae the PTU Then i changed out the IAC ran fine.! Clean out the intake and throttle body with the best carb cleaner. BUILD up from recirculating gases emissions causes black residue on the throttle plate behind it and around the edges. Make sure its clean and free of any debris. and use only MAF sensor cleaner to clean out the MAF Dont mess with any screws on the throttle body except for the screw on top. Thats for the IAC it can be adjusted in making it idle low and screw out idle high. There is another set screw behind the throttle body near the lever just under it. Dont mess with that! Its for the throttle intake and its set from factory specs. And keeping it from sicking shut if the gas pedal is released.