2.2 idle problem...
 

Ok, I've done everything I can think of, but my car still has an idle problem. Car is a 97 Impreza L with a stock EJ22 with about 234,000 miles on it. Here's what's happened so far...
2-3 months ago, I was in Yosemite and stopped for about an hour to get lunch, when I started the car back up to leave, the idle shot up to about 2,000 for about 5 seconds then sounded like 2 cylinders cut out, and the CEL came on at the same time, then it died. . Tried to restart several times with no results, then waited a few minutes. Tried again and the car started up and idled fine, but the CEL stayed on for a couple of days. Got the car home and used the code reader and got either code 1501 or 1507, can't remember which one...in any case, it was IAC valve malfunction. After the CEL went off, I didn't have any problems until a couple of weeks ago, when the car (after it was warmed up) would idle fine at about 700 rpm, then would drop down and almost stall, then back up to normal. This would happen regularly every 30 seconds or so.
Yesterday I pulled the IAC and replaced it with one from a spare intake. I thoroughly cleaned that one before installing it. Now the car idles about 50-100 rpm higher than it used to, but still has the rythmic near stall out every 30 seconds or so. I don't know Subaru engines nearly as well as I could, so if anyone has any sugestions, I'd really appreciate it.

Vacuum leak or O2 sensor.

Spray carb cleaner around and find the leak I believe is the Cory approved method.

This is my bet.

Yeah. Maybe spend an afternoon replacing all hoses associated with the intake manifold, and changing the intake manifold gaskets and throttle body gasket. With the age and high miles, there is probably more than one issue there.

My 22 did the same thing and the issue was a vacuum leak.

I had this issue on the RS...I ended up cleaning every part of the IAC that I could...After that it worked just fine...