I've been there too... takes quite a set for you to post it here. I had a bout where my heart would race, to the point I thought I was having heart attacks (I'm 35). Once, while working on-site (software developer) working 15 hour days, I od'ed on caffeine, and skimped the sleep. When I went to bed, I was ripped out of bed with my heart trying to leave my chest. I was hospitalized, spent days in observation only for them to find that my heart was fine. The stress pushed me over the edge into a panic attack. For days, I would leave work (still on-site, hundreds of miles from home) I would finish my day, then drive to the ER, just in case it happened again. Even slept there (in my car) one night. I hit the ER 3 more times, and finally, met with a doctor with a brain.
The best thing that helped me was that I described my symptoms to a woman I work with (CFO at a customer) and she said she had that symptom all the time... comes when you are most relaxed... perhaps when you are trying to sleep. Heart racing (and hurting, like you just ran 5 miles), sweat, nausea, etc - basically like you're dying!
Anyhow, a very small dose of Paxil helped me. I went for almost a year taking it twice a day. Be prepared for some very uncomfortable side affects while you acclimate, but once acclimated, you're just a little tingly/numb in all the wrong places
Anyhow, it's been a year this August since I went off the drug, with no relapses. I now run 2-3 miles every other day (See what Bill and others wrote above) and drink NO caffeine (really can aggravate the situation). Anyhow, sorry for the long post, I thought my story could help; feel free to PM me if you'd like.
-DMA