OK, I'm kind of confused. You said your original fast idle was around 2000 and you didn't like that. Eventually when your engine warmed up it dropped to ~900 rpm. You put your newly-acquired IAC in and now the fast idle is at 1500, which sounds about right to me. Did you let the engine warm up and see what the idle was when warm? It should be around 700 - 800 rpm.

In my '89, when the engine is cold, the fast idle is around 1800. It drops down gradually as the engine warms up. It should take your engine somewhere around 5 minutes to warm up to the point where the temp gauge is beyond the low line. If this is not happening you most likely have a stuck thermostat. Bottom line is that when the engine is cold, the idle is supposed to be high.

Eric W.