That's a honda quirk all of them do it. I believe a lot of it has to do with carbon build up. It's always a good idea to run a fuel injection cleaner every once in a while when you get your fill up on fuel to keep your injectors and cumbustion chamber clean. You may be able to kick your idle adjustment screw up to 900 rpms and smooth it out a little, but usually that's all controlled by the computer, so if running fuel injector cleaner doesn't help overtime and a tune up dosen't do much then your going to have to look to sensors that give refrence to the I.A.C valve.