Rpm 2500 to 3000 when you start then goes down 1500

In 99% of the cases of GMs doing that it's the Idle Air Control motor, which you already changed. But changing it does not always fix the problem. First, if you bought the new IAC motor second hand, change it again cause last time i had to change mine 3 times before i got one that worked OK. Second, if it was bought new and/or your are 100% sure the IAC is working fine, then you probably just have to much carbon deposits where the motor's tip and the intake manifold meet and are supposed to seal up, preventing it from closing normally and giving the engine too much air, cause the high RPM idle. So take the IAC out again, clean the opening, lip and everything you can see and get into around that area, with a cue-tip and some contact cleaner, or lighter fluid or rubbing alcohol to get all the carbon deposits out and you should be ok. If not, then the problem probably resides in the car's onboard computer.