GTS-4some, First up, I would pull apart the IAC valve located near the throttle body and give it a clean. This controls your idle speed. If the IAC valve is fine I would then go onto the sensor that controls this. Not directly but the ECU input would be the 2 pinned coolant temperature sensor. Located in the water jacket normally 9 times out of 10 up the front of the engine.
As for Tommo's problem, I would say that if the coolant temp sensor is not telling the computer that the car is now hot, you will still have rich mixtures and this will kill an exhaust in no time. You will find that you will get poor fuel consumption and be down on power.
Cheers
Sumo
P.S. Im telling you what I know in my universal applications of how cars work, I have had bugger all expirience with skylines in particular, but once you get your foundations straight you can normall apply this to most cars.