Problem with idle and tranny shift

Hi
I have a 2001 altima and the cars idle is at 1075rpm. That is not normal but it gets worse. When I am on the highway doing about 70mph the rpms are at 2800. When you first start the car in the morning the car goes thru the warmup process ok but if you run it within the first 15 minutes and put the car in nuetral or park it idles correctly at about 650-750rpms. Once the car is fully warmed up the idle returns to the incorrect 1075 or 975 really depends on the car I have no control. My main concern is possible tranny damage as a result of this or other problems. I have never had the check engine light come on since I've owned the car for 3 months. Any and all suggestions would be appreceiated. I have called the dealer and they give me the you'll have to leave for a few days which means big money and no car which I must have to go to work. I am a huge fan of the dealer when I know the problem is evident but in this case I would proably be better trying to fix this my self first if at all possible.