I'm 29 and have experienced the same symtpoms.  I could be sitting there reading a book and my heart starts pounding, thumping almost and the heart rate is through the roof.  It literally takes my breath away and makes me lightheaded.  I have to ride it out for a few minutes and then I'm fine.  It doesn't happen often, but it's scary enough to warrant concern.  I work out and eat right and am within my weight range for my height/age.  My doctor thinks it's stress related as well, but frankly, I'm tired of that 'diagnosis'.  I'd love to hear if anyone has any other ideas other than stress before I drop dead of a heart attack and an autopsy reveals something else.  I tend to have low blood pressure if that helps????