My five month old puppy, Dylan, is a sweet and loving dog. He's very well behaved and listens when we're inside the house. He's even walking better on leash. However, when I take him out in the yard to play (tossing his favorite outdoor toy) and generally letting him run around, he becomes very aggressive. He barks at me, growls, and nips at my ankles. Doing some obedience work seems to snap him out of this aggressive mind set for a little while and then the behavior starts again. Any words of wisdom would be much appreciated.