With turbo charged engines it helps to run one heat range colder plug and a larger gap. No more than .010 larger. The larger gap helps the idle because turbos tend to run pretty lean at idle even though there isn't any boost yet. The colder plug helps above idle especially if you do a lot of hwy driving or hard driving around town. If you have a bent or burned valve it will idle very rough and slowly smooth out as rpms increase to about 3000. As usuall check for vacuum leaks. However try going to an Impreza web site. A good one is NAIOC.comNorth American Impreza Owners Club. There you will talk to people who obviously own the cars and probably have come across this problem. Good Luck