What Causes Frequent Attacks Of Vertigo?

Hi. I am a 41 yr old female in pretty good health. For the past year and 5 months, or so, I have bill and tired. Does anyone have any answeseen having frequent attacks of vertigo. This is a sensation that comes on while I am asleep or awake, where I loo,se my balance, things spin, I get nautious, sometimes vommit, and my blood pressure drops after an attack. These attacks happen about every four days or so, and last approxamately 20 minutes. I have now seen five doctors, including my gp, two ents, a neurologist, a chiropractor, and an endocrynologist. Mri was fine, eeg was fine, eng was fine, cortisol levels were fine, and it doesnt seem to be positional either. I am currently awaiting results of aldosterone level test. I am feeling quite hopeless as to _finding help or get relief. These attacks leave me feeling ill and tired. Does anyone have any suggestions?