could be springer rage,my mates just had his put down at 8 years old.it started like yours a couple of year back then it started biting and then snapping at people and the last year it got really bad going for everyone even kids locking on and going for the throat and it wouldnt know it even did it tail wagging and all that,might not be like but thats what happend to his,so i googled it and it said springer rage.