The first thing I would check is the throttle valve switch to verify that the idle contacts close when the throttle is in the idle position. If the switch misbehaves the ECU will not go into idle stabilization mode.

A flaky switch is usually just a matter of doing a thorough cleaning on both sides of the throttle valve shaft and checking the elecetrical connector on the TVS pigtail for good contact with it's mating connector on the engine harness.

Duke