Opinions about episodes of incoordination in my pooch, Max?

Max is a fifty lb black and white mutt who lives with us on a mountain at 7500 feet near Medellin, Colombia. He is a healthy, active dog, but has experienced three separate spells where he is uncoordinated, unable to walk, and bangs into furniture. These episodes last 1-2 minutes followed by panting, trembling, clinging behavior, and drooling. He recovers spontaneously and after another 10-15 minutes calms and is himself again. A visiting vet witnessed one of these spells during a stressful house call and opined it was a stress reaction, but Max has had two other spells over the past five years when he was calm beforehand. He is a little overweight, has been treated for low thyroid levels, has a fatty liver and has high cholesterol, on a special diet. He is healthy otherwise, active, and lives on a 5000 square meter mountain side lot. He gets lots of exercise and has a housemate, a mini Labradoodle named Ozzie. These spells don't seem like a focal or grand mal seizure, but I am at a loss to explain them. Any opinions would be most welcome.