I just finally fixed a huge idle issue on my turbo 2.0 which was very lopey and unstable bouncing between 200-750 etc etc almost as if it souned cammed up, also smoked black smoke as if it was running rich.
Problem ended up being the exhaust cam was out of time by 1 tooth. So it couldn't hurt to double check your timing marks on the motor.
Simpler things to check would be IAC motor,and TPS sensor make sure they are both functional. They both adjust the idle and stability of the air coming through the throttle body into the manifold.
I assume you already did the "maintence" thing like you said, plugs, wires, air filter, fuel filter, oil change etc etc.
Could also check fuel injector o-rings. Im not sure how the PCV valve effects the non turbo 2.4 when its not functioning, but on my previous cars that were non turbo, a bad pcv has caused me some idle issues. Just some ideas hope you have some luck with it bud.