Irratic idle and such

Hello,
I have owned a 1991 Firebird for about 2 years now and have been having this idle problem for as long as I can remember. Basically, when you first start it, it idle's really high like up to 1800 RPM sometimes. Eventually it will go down but then the problem is the idle is very irratic and when I am at a stopsign for example or a redlight just sitting there the idle will fall and the car will just stall. Most of the time it high idles. And like I said the idle is just so irratic. So far we have had the timing checked (was off by 10 degrees but fixed) put a new ECM in, put a new ISC in, and a few others things I can't remember right off the top of my head. We've also noticed the engine running very lean lately. I also was thinking it might be the IAC valve. Anyhelp would be so appreciative this thing has been a pain in my butt from the time I bought it.
thanks,
Steve

Welcome Steve and the IAC would be my first guess.

thanks alot!

Turns out I already replaced the IAC valve and apparently forgot. Because I went to go look at it and it looks brand new (and I also remember putting it on now) so I guess that isn't the problem with my irratic idle. But I was just doing some more research and I was reading a bad EGR valve can cause a rough idle. Anyone know if thats a possibility?
Thanks,
Steve

Don't know about the EGR valve, I had a similar problem with my 89. Turned out to be the coolant temperature sensor. It was getting no low temperature input, so the ecm revved the engine really high, thinking it was very cold out.

take some carb cleaner spray it around ur intake, and throttle body or injectors or whatever u have. if the idle goes up u got urself a vacum leack cuz vacum leaks are known to affect idle like u describe.

Currently my car has an irractic idle. Its probably a crack in the intake. Or atleast that is what I was told.

when you put your IAC in did you set the plunger to the specified postion, or length? You may need to get the specs and reset it.