How do I fix an idle problem with an 89 Firebird 305 v8 TBI?

I have a 1989 Firebird 305 V8 with throttle body injection. When i start the car it idles very low for about a second, then the service engine light comes on and it idles really high; around 2000 or 1500rpm. Also when it is in drive with the light on it sometimes will die while stopped. What is causing this and how do I fix it? I have replaced the idle air control and throttle position sensor.