1987 olds calais

Hello, I Have A 1987 Olds Calais 3.0 Ltr V-6 Auto, When I Start The Car Up First Thing ,it Runs At A Very Fast Idle 2000rpm, Doesnt Kick Down To A Lower Rpm, If I Let It Run For A Short While 5- 10 Minutes Then Shut It Off Wait A Minute Or Two Then Restart It Runs At 1200 1500 Rpm,if I Do The Same As I Did The First Time Then Wait A Short While And Restart It Runs At700 -1000 Rpm What Is Causing This ? When I Let The Car Run For A 15-20 Minutes At The Lower Idle 700-1000 Rpm The Car Dies And Wont Restart Right Away If I Let I Sit For A Short While 10 Minutes Or So Then It Restarts Anyone Heard Of This Or Had It Happen To Them? Its Driving Me Crazy Any Help Would Be Greatful,,,,,,,,,,,,,,,,,,,,,,,thanks Alot