My dog has been losing his balance when in a relaxed mode. He also has been breathing with his mouth open and has been breathing really heavy. He seems to be very exhausted when he relaxes but doesn't want to lay down. It seems like he try's to sleep standing up but then loses his balance when he doses off. I have been noticing symptoms for the past month that have been progressively getting worse. He is a 4 year old chihuahua that weighs about 8lbs. I have had two different Veterinary doctors look at him and they can't seem to figure out whats wrong. They have done blood work which has come out fine. They have done x-rays and have found nothing that explains his heavy breathing. We just ran CT scan which a neurologist reviewed and has found nothing. Everyone has come to the conclusion that he is not okay but they can not figure out whats wrong. He seems to be fine when active (will still jump up and down when I come home, chases squirrels, begs at my lap for human food) he does seem to becoming less active as the days passed. His latest symptoms is that his legs are now buckling. I fear that I am losing time, and nobody can help. Any suggestions on what might be causing this?