the solenoid in your throttle body that regulates the idle, is dirty or seized, and needs a good clean or if its really bad replacement.
(its a little bit ontop of your throttle body, (coolant runs to it on mine) take it apart and clean it/unjam it/replace it. but its kinda hard to explain, and im at uni atm so i cant get a picture, but yeah, if you keep driving around, it might vary heaps, from normal, to a little bit of antilag spec, to the point were its nearly undrivable, lol and you might get an engine light)

subaru will tell you that for 147, and fix it with a new one for ~700 lol.
second hand ones are about ~400
but i bet if you take it to decent garage, they can sort you out.
maximum helped me out
i put a thread up on here somewhere, but i couldn't find it, but yeah if you still need help, pm or 0437380338