Hey Guys,
This is my first posting on this sight so I hope I have this right.
As it just happens I just had this happen with my 97' Outback. The rough idle can be caused by carbon and gummy oil deposits in the throttle body and on the throttle plate. These deposits don't let the plate close completely resulting in rough idle. Simply remove the air intake, and using your room mates tooth brush and spray carberator cleaner, scrub out the deposits on both sides of the plate and all around the opening. Caution! don't use too much carb cleaner all at once. I flooded mine and had to remove the plugs and dry them before it would run again. But the idle was back to silky smooth!
Hope this helps!
Subernut