We recently got a rescue GSD who is lovely in most way but every now and then for no apparent reason he will attack one of the other dogs and not just a warning to them but a full blown attack. When it happens it is not over food or toys it just seems as if a switch goes in his head and he goes into attack mode. Is it worth getting a behaviourist (sp?) in, has anyone else had this problem and what did you do to resolve it? Any help would be really appreciated, thank you.