Fainting Spells, Done Blood Work, Physical Exam, Had High BP

Question : I have gone to two doctors in two different countries concerning periodic fainting spells. What normally happens is I will be doing some ordinary task either sitting down or standing up with no sudden movements and I will start to get very tired, then I will get hot, dizzy and I will go completely pale. Anyone who looks at me in this state can tell there is something seriously wrong. If I am not able to "rest", I will faint. Usually, I will only faint if I am standing up, but it has happened once when I was sitting down. It has happened both before and after eating. Both doctors I have seen have done blood work and physical exams but nothing unusual comes up. I also get a physical exam twice a year for general health (not for this condition). I am 25 years old, but do have high blood pressure. The only explanation I have been offered was stress, but my American doctor admitted that he was not sure of the problem. Anemia was also ruled out in the blood work, though that seemed the most likely culprit to me. Stress is also unlikely as it has happened to me at parties, on vacation, at work, walking around the countryside, relaxing at home and it has seemed most likely to happen when I am very happy with my life.

I would really like any advice to help with this problem. It disrupts my life greatly and leaves me feeling extremely lethargic and "shaky" for a couple of hours.