I have had the same problem for around 20 years now. Now matter how many test are ran and doctors I see they find nothing. I went to a Chiropractor and she said she could help. I didn't believe her and I had past out again. I went to see her and she found a nerve pinched in my neck. I go back to her several times a year and it seems to help. I have episodes when I don't go see her more often. Not sure if the neck is what is causing it or not, but I have been going to see her for around 18 years.
Remar brings up a good point regarding blood sugar. I would also make sure you're keeping yourself well hydrated.
This really does'nt sound like anxiety to me. It could be though because anxiety comes with a very long list of symptoms. I do think fainting would be a rare one though. You've tried antidepressants and there was no change. It really does make me wonder if this is anxiety because if it were the AD's should have helped.
Have you had your blood sugar tested? If it gets to high or to low you can faint.