We have an 8 month old, Neapolitan Mastiff bitch of the working variety. Unfortunately, we seem to have encountered a serious problem when my fiancee and I take her for a run. Out of nowhere she will start by sprinting in circles around us, ears down, eyes in the back of her head, growling and then jumps up on us and then nips quite hard (sometimes drawing blood) and still growling. We have tried everything to stop her, from squirting her with a water pistol, smacking her, commanding her "down", turning our back and grabbing her by the scruff of the neck, none of which have remotely worked. I would like to stress we never encourage her to jump up or rough house with her. We can't seem to find the stimulus for this behaviour, can be off lead or on, anytime of day, no sign of any other dogs or people. It just seems to be excitment overload as it only ever happens on runs in the woods, never at home or in the garden. We would really appreciate any feedback or advice as my fiancee in particular finds it very frightening.