A 63-year-old woman has three sudden episodes over two days of dizziness followed by double vision, slurred speech and veering to the left; each episode lasted about 10 minutes. Between episodes she is neurologically normal, and there is no headache, positional trigger or hearing loss. She has hypertension and smokes 15 cigarettes daily. During the latest episode, her partner noticed that both eyes appeared to move normally, although she could not keep objects single. Which one of the following is the most likely diagnosis?