I have a 4 month old weimaraner/terrier puppy and he's driving me crazy! One minute he's perfectly fine, calm, a good puppy, the next minute he's jumping on my lap growling and barking at me. His tail is wagging but he's baring his teeth. Then, he gives up, walks away, and gnaws on his toys. We walk him at least 2 hours a day (sometimes about an hour and a half) and he's not normally locked up. What do I do? It really does scare me when he snaps at me like that, and I try to keep the whole calm assertive attitude, but when he catches me off guard by barking, well, it scares me. Please help!