A 40-year-old man reports episodes in which he unexpectedly feels a surge of intense fear that peaks within minutes. During this time he feels dizzy, has shortness of breath, heart palpitations, and feels like he is "going crazy." These episodes seem to come "out of the blue" with no trigger that he can identify. He has had them when he has been napping on the weekend or during a break at work. He is starting to think he is really "going crazy," because he has no medical conditions, doesn't drink or smoke, and says there are no new stressors in his life that could be causing this. What is the most likely diagnosis?