Go see your own doctor. If you don't get any answers, see someone else. As to the possibility of losing your job, depending on what you do at the foundry (I read your comment to someone else's answer), you could be endangering the safety of yourself or others when you pass out. Ask to be reassigned to a different department or to be placed on a medical leave of absence until you can get this problem under control.
Search online for medical diagnoses, talk to online doctors, see other doctors, but don't give up searching. Have you ever watches Mystery Diagnosis? Doctors can easily over look something, thats why you should always go see multiple doctors, and get other expert opinions.
if you lose your job at a hospital because of passing out on the job and they do not care sue there assses and dont go there for medical attention of any kind this is illegal they cant fire you they have to put you in another department.