It sounds as if the base idle isn't set correctly. With the idle speed control valve disconnected, the engine should run at 600 rpm. There is an idle air bypass screw on the throttle body, facing forward, next to the throttle position sensor.

It could also be the vehicle speed sensor not working, this tends to allow the revs to drop when you pull up to a stop. On a RWD car, this sensor is located on the side of the gearbox, near the back. On 4WD versions, it is at the bottom of the speedo cable where it joins the intermediate shaft through the sump.

If you lift off the throttle, the fuel is cut off above 1700 rpm, to reduce emissions and improve fuel economy.