You may need to clean your Idle Air Control valve. I had the same issue and solved it by cleaning the valve.
Mike
A few times in the recent past (it doesn't happen all of the time), I've
>had a situation where I'd coldstart my 2000 OBW in the morning and it's
>idle speed would be 3000rpm! Now this has happened a couple of times
>despite the fact that it was sitting in a heated garage overnight. It also
>happened another time while it was sitting outside in relatively mild
>weather (for this time in Canada) of just around 0 Celcius. In the past
>I've seen Subaru's variable fast idle speeds never went over 1500 or
>2000rpm, even in the worst of conditions (i.e. sitting outside at -20C or
>lower). I changed the air filter since it was dirty, but it's happened a
>couple of times since the air filter change too.
>
> During the times when it's idling at 3000rpm it's keeps tending to want to
> keep at that speed for what must be 5 minutes, even while driving. So when
> shifting gears (i.e. 5MT), you have the unusual situation where the engine
> speeds up between shifts rather than slows down.
>
> Computer problems? Sensors?
>
> Yousuf Khan