Q: Engine hunts/stalls at idle.

I am having an issue with the engine hunting while at idle, sometime causing it to stall out. On a cold start, engine revs to about 2000 rpm then slowly lowers to approximately 800 rpm. As the engine continues to warm up it starts to hunt at idle. If driving, it will sometimes cause the engine to stall out when coming to a stop. I have just had the engine replace due to another issue but this problem began before I swapped out the engine. I took it to a ford dealer and they said that I have a worn IAC and and intake gasket(s) leak. I have replaced the IAC valve (easiest thing to do). But I find it hard to believe that the intake is leaking because I think it would hunt at all RPMs. I also notice that it seams to worsen when electrical system is under load (e.g. turning on AC). I know that there could be several causes for this but it's a little overwhelming and where do you start? I don't want to throw a lot of money at issues that aren't issues

My car has 86789 miles.
My car has an automatic transmission.