In another thread you mentioned using Autolite plugs. I haven't personally used this brand in a Subaru, but I'll tell you this: When I bought my car it had Champion plugs in it. Didn't idle very smoothly. I installed some Bosch platinum plugs. Big mistake. Something about the Bosch plugs (and this is well documented in the service and maintenance forum) makes our cars run like crap. I had a lot of hesitation, and an extremely rough idle. I pulled the plugs, check the gap, check all my plug boots, still had the problem.
I swapped them out for NGK V-Power copper plugs, and it ran perfect. I use these plugs exclusively now.
I use Bosch plats on my Ford truck and love them, but they're just no good for our motors, IMO.
So, perhaps try using a different plug. Did you replace the fuel filter? I agree with cleaning the IACV.
Are 2002 vehicles MAP-based? If not, you can try cleaning the MAF sensor. Lack of power can be caused by running a little lean due to a dirty MAF.