Sudden drop in blood pressure

I have been living with sudden drops in my blood pressure that causes me to pass out, for the past two years.  My blood pressure is normally high and I am on medication.  I have falls too numerable to count.  Doctors have run every known test and cannot find the cause.  I have had heart catherization, table tilt test, 3 day EEG studies, holter monitor test, ecocardigram, MMR's, MRA's , doppler , you get the picture.  The only thing that they found was a blockage in one of my carotid arteries.  I had surgery last week to correct this, thinking that this would be the answer, but just this week I have the same problem.  I have fainted and had three more bad falls since the surgery.  Can anyone help me?  I am Desprate.