Bassett hound freaks out

Our dog is healthy and happy, usually. He's been to the vet once per year to get his shots and a check up, once only 2 months ago. He dprained his tail when my partner and I were both away from the house one evening and we took him to the vet the next day and he was fine within 3 days. My question is about his behavioural outbursts. Once in a while he has outbursts where he will run away from everything and everyone barking non stop and howling. we cannot regain control over him during these eppisodes, and its really frightening. It has happened 4 times in the past 2 months. There is no common trigger and we cannot figure out why it happens. The first time, he was in our house with another friends hound and out of the blue started freaking out and ran round our neighborhood barking and freaking out, the second time we were on a bike ride with some friends and other dogs, the third time walking home from a friends and the fourth time today at the lake. He would not let anyone near him and ran way off into the woods. Its not a regular barking either, he seems very disstressed and scared. Can you please suggest what may be causeing this behaviour? PS. there is never any injurry site, no licking or biting anywhere and after the eppisodes he calms back to normal within 20 minutes.