[masraum]

Well, when you put the car in drive (vs N) that puts a slight load on the engine which usually also makes the idle drop. I think there's supposed to be a solenoid that bumps the idle up a bit (or is that just when the AC is on?). As asked previously, what are the rpms in N and what are they in D? Do you run the AC? If so, test in N and D with the AC on and then again with the AC off and see what the results are. If you never run the AC, you still may want to test.

It does seem odd that it idles better cold than warm, unless it is a vacuum leak, in which case it kind of makes sense that it's better cold than warm. I know these things don't have a choke like a carb, but I'm sure the computers make the mixture a little rich when the car is cold to simulate a choke. If you have a vacuum leak, that may bring the mixture back to something approaching normal due to the extra air that's getting in due to the vacuum leak. At least, that sounds reasonable to me.