A 55-year-old man has had bouts of severe dizziness, lasting a few minutes each time, for 10 days. He feel nauseas and has the sensation that his surroundings are spinning. These episodes occur with changes in posture and he has attacks up to ten times a day. He had a viral illness about 2 weeks ago. He has no nystagmus, Romberg’s sign is negative and his ENT examination is normal. He has a positive Dix-Hallpike test. What is the most likely diagnosis?