good day to all.

I'm glad to have this site visited where I can get know-how as I am more a D-I-Y guy as far as my car is concerned. My car is an old automatic Nissan Sentra 1991 and I have a question regarding the drop in my RPM when in idle. I noticed that my rpm falls to almost 500 (from 1000) when I release my gas pedal creating a choking effect turning off of my engine ( in the middle of the road and there's a queue of cars behind me). My problem started when I tried to adjust my rpm. When I experienced an engine stalling. I returned it to the old rpm but to no avail-my car still stalled. .Anyone who can help me?