A 42-year-old male presents with recurrent substernal chest pain over the past year. The episodes are unrelated to exertion but often occur when walking against cold wind or at 5 AM. Pain is relieved by nitroglycerin. Baseline electrocardiogram (ECG) shows horizontal ST-segment depression of 0.75 mm in leads II, III, and aVF. During episodes, the ECG appears normal. The most likely diagnosis is: