A few months back I got 03 WRX. I love the car, and miss it badly at the moment (currently on deployment , in the Navy). I ran into a slight issue right before I left, so I didn't have the time to fix it, but at the same time im not fully sure what's causing the problem.
My friend, a bit more knowledgeable with turbo'ed cars, somehow he pulled a line somewhere, probably trying to see something and pulled it and forgot I guess, and later on I was able to find the vacuum leak. During the leak the car would idle horribly at a dead stop, to the point that sometimes it would shut off. Very embarrassing.
However, after it was fixed I still ran into one more issue. At a dead stop, the car idles fine, UNLESS the radiator fan starts to kick in, or I hit the AC button. Not long before this I could swear I heard a slight squeak whenever the AC was on while driving. AC works fine though. When the radiator fan kicks on the idle will go extremely low, then jump back up, then back down, and sometimes it will be enough to shut off the car. The AC button will almost shut the car off during a stop every time within several seconds.
I know the dealer I bought it from had to replace the power steering belt before I bought it because it was worn pretty bad. I didn't really have enough time because this was 2 days before deployment, but I'm assuming the belt for the alternator may be worn? In the very beginning during the vacuum leak I did on search on this forum and though it may have been the Idle Air control valve, but I don't believe that could be the case anymore.
Any other ideas on what this may be?