A 59-year-old man suddenly develops difficulty speaking and weakness of the right arm while driving. His wife reports that, immediately beforehand, the right side of his face and arm jerked rhythmically for about 90 seconds. The weakness and speech difficulty resolve over the next 40 minutes. CT shows no haemorrhage or established infarction, and his capillary glucose concentration is 6.1 mmol/L (reference range 3.5–7.8). Which one of the following is the most likely diagnosis?