B1900 wanders when on "app" mode

I am flying the b1900 as a regional airliner, but, for some reason,its ils lock in behavior is outright crazy: the plane keeps on chasing the needle--not matter how far away from the airport I lock in the ils. Why does it overreact so much, and I do I get into it to fix it? The plane otherwise handles nicely and, manually, it's a pure joy.tony