From a cold start, it runs just fine for a couple minutes, after that, as if someone threw a switch, it goes into another mode and I get (what seems to be) a misfire. That is, it stutters when I try to rev it up in park, while in this "warm mode".

Additionally, it idles steady in park at around 1800 RPM, if I don't drive it and just let it warm up from cold. This differs from the up/down idle (between 0 and 1000 RPM) in park or neutral that I get after the car has been driven and fully warmed up.

I dunno, it's strange.. This is about as accurate of a description of the problem that I can give. Hopefully someone who has had a similar problem can help as I've got to get this thing smogged soon.