Why can't l maintain proper RPM of 850-1000 when the A/C not running?

L have to increase my idle on a daily basis, or keep the A/C on to prevent the engine from shutting off. With the A/C off the car will cold start fine and idle just above 1000rpm. Once it has warmed up, the idle goes below 500rpm and engine dies if l shift into reverse or don't press on the gas. When in drive, l must shift into neutral to make stops as l have to keep pressing on the gas or the engine will shut off.

With A/C on, rpm fluctuates between 850-1000; driving and stopping is normal however, when reverse gear is selected, the rpm goes below 850 but not too low to where the engine will die. L have replaced the air filter and removed and cleaned the throttle body but the problem won't go away,