Also, ignition timing is PART of the timing belt change procedure... so why are you trying to troubleshoot before you finish your timing belt job? Adjusting the idle screw is also technically part of this.
Unplug the o2 sensor and see if the idle changes. Also, it is safest to unplug or plug in the ISV with the battery disconnected as far as I have heard (though others have told me that the key just needs to be out, but do you really want to gamble with the ECU)?
A bad G60 could add drag or reduce the air in the mix and create a weird idle until the computer kicks in after it reaches temp... I suppose. It would have to be pretty shot though.
Keep in mind the computer is basically blind and deaf on a cold start, so it takes certain conditions for granted. It is programmed to be generous in order to get your car the fuel, etc that it needs on a cold start, even if your car is a bit off or you are in a weird climate. However, it is still assuming that you are not going to exceed those maximum or minimum variable values that are programmed into the mapping for the cold start. If something is pretty far off, it might have a hard time on a cold start but then sortof recover once the sensors are all working together to make adjustments, though they are maxing out their adjustability so to speak. I just had this type of situation happen with my AFR, it would only normalize once the sensors were warmed up and the computer knew to basically open everything up all the way.