What could be the problem of my Nissan Sentra.

What could be the problem of my nissan sentra 1997(170miles) automatic

1. When I shift the gear from neutral/park to primary it does not bite right away as it used to be when I hit the gas, it takes 10-20sec before it kick. But if I shift from neutral/park gear to secondary or drive right away it kicks to run.
2. When the car is in the neutral, the reading is in ".9" but why when the aircon automatically turned on, it drops down below ".5"and hit back to.6-7 rpm reading which make the engine to surge and run slow even if I hit the gas more, the car still hardly pushes/run as fast as it should be. Esp when I hit the brake to stop the car the rpm suddenly drop down below.5 more and it even makes the engine to shut off.
3. The aircon, I have it cleaned, the alternator was checked, the air sensor was cleaned and were replaced the last time. Oils are newly changed, sparkplug/tune up was done. For a while the problem stops but for only in a week time the problem comes back again.
pls help, i've been suffering for more than 2 yrs already having it repaired to many mechanics but they were not able to really fix the main problem.