Low Idle - Need help!

I have a 2000 Altima... it idles REALLY low when I'm stopped and driving slow (like below 500 RPMs). I have to throw it in neutral and give it gas to keep it from stalling at lights - every time. It gets bad gas mileage, and when I floor it, the temp goes WAY down. Can anyone help? I've changes the spark plugs, and someone said that it might be the throttle body, or fuel injection... could either of those be it?

mmm.... It might be your MAF ( Mass Air Flow Sensor ). Mine was doing the same thing, if you rev it too high then its starts smoking a lil ... the MAF is about 500 from nissan... i got mine from a junk yard for about 170. I really dont think it might be the injectors.... though it might not be a bad idea to go ahead and get the injectors cleaned... but i really do think its the MAF...