I have a 305 TBI 89 Firebird. When I start the car it'll idle very low, around 500-700 rpm for about a second. Then the service engine light will come on and it'll idle up around 1500. Also when in drive and I stop the car it will sometimes die because it idles extremely low again. I've replaced the throttle position sensor and idle air control. The car just started doing this randomly one day.