I had the same problem. Started at about 18 when my parents got divorced. I've had heart monitors, head scans, ENT probes, blood tests... nothing at all physically wrong. They're like clockwork now whenever something that I'm not comfortable with happens. I can handle big meetings at the office because I've done them so much, but 1x1 with a new VP or perhaps driving into a new part of town that has very heavy traffic... that will trigger it.
I've learned to deal with them by taking a few deep breaths, remembering it's all mental, and through preparation. Also, whenever you're going into a stressful situation. PREPARE. PREPARE. PREPARE. If you know you're heading into unfamiliar territory or you know you're heading into a situation that may cause the problem, be ready for it. Review it in your head a few times. Plan out how to handle the unexpected. Not only will you remain much more calm, you'll simply handle any situation better and more thoughtfully.
I've taken medicine for them in the past. I've got it pretty well cracked now, so I haven't taken any in a while.
-Jeff