Hi there,
I assume from the way you write, that your epilepsy test came back negative? Also, you mentioned that your blood pressure is up and down frequently. Have doctors attempted to control the blood pressure with meds? Have you had your blood sugar tested? If you have diabetes (or dont eat for long periods) that can cause fainting. Also some meds can cause fainting as a side effect since they alter your blood pressure. One final thought... Does it happen when you turn your head? Sometimes the neck bones can restrict the carotid artery in your neck and that can cause fainting.
Like Carla, I can tell that this is driving you crazy and I wish there were more I could do to help.
Darren