If, by chance, you let your car's battery die entirely, I have heard of occasions where your car's computer becomes disorientated, or reset if you will, and your idle can get messed up. what i would do is avoid letting the idle drift too far below 750 RPMs and your computer will eventually re-learn what your proper idle is over time.
It keeps popping out of auto "WHY"
Overload or short in wiring?
It is supposed to keep idling when you put it in park, unless you turn the ignition key off.
That's almost a question.
the car is a 1990 deville
You have a virus. Or it is continuously overheating?
Sludge
Yes, it should stop all shutting down actions by your computer if the system is overheating.
Fuel pump or filter??
change MAP sensor
if fans are working and thermostat is good welcome to the world of northstar famous headgasket/faulty headbolt and stripped block crap engine problem
format disk drive C: / ,theninstall a fresh OS