The problem may be with your Idle Control Motor. On these older EFI models, the Idle motor or solenoid must be calibrated to a minimum and maximum idle speed. Indiscriminately turning the hex adjusting screw on the Idle motor will cause symptoms as you described. Place transmission in NEUTRAL position.
Apply parking brake and/or block rear wheels.
START vehicle and allow to run for at least two minutes.
Turn engine OFF, and allow 60 seconds to elapse for the throttle kicker to fully engage. Disconnect the electrical connector to the idle speed control actuator. Disconnect the electrical connector to the coolant temperature sensor. Connect a tachometer to the engine.
START the engine and allow to warm up to normal operating temperature. Adjust the extension screw on the idle speed control actuator to 2500 to 2600 rpm. Turn the engine OFF.
Reconnect the electrical connector to the idle speed control actuator.
Reconnect the electrical connector to the coolant temperature sensor, and erase any fault codes that may have been set. In addition, check and replace all vacuum hoses that are cracked, split or swollen,