My Forester has about 114k on it. I am having a problem lately when ever I start it in the morning only in the morning after sitting all night. I will start it and it revs up to 1500-2000rpm and stay there for half a second. After that it will drop to like a rock to 500rpms and sputter and start to die then it recovers and its fine. No Check engine no nothing once its done its fine. Any Ideas before this gets worse?