Whats should the rpm on a 1999 infiniti q45 be when the its on and its on and on P?

hello, i have an automatic1999 infiniti q45 and im having a bit of a problem with it. When i start the car up the RPM seems to be a little low when its on and parked, it reads about 800 rpm, but when i put it on reverse or drive it drops real low to almost 500 rpm and it feels like its gonna turn off. once i drive it around for a while it goes up a little bit, when im at a stop or a red light it reads about 700, and when i park it after driving it, it reads 800 but if i accelerate it its goes up to 900 and stays there, i wanna know what i can do and if thats the normal rpm for my car. i had the MAF sensor cleaned thinking that was gonna be it, but it didnt help. Would adjusting the RPM to 1000 help. i would appreciate your answers =)