Starburst and SylvieM, you might consider testing for Autoimmune dysautonomia/autoimmune autonomic ganglionopathy. If you got it you can get immunological treatments (IVIG, Rituximab, etc.) that might help to improve the damage and not just to alleviate the symptoms (Florinef, Midodrine, etc. ).

There is a great lecture about it here:
http://www.mayomedicallaboratories.c...12/07-autodys/

The main test for this dysautonomia is the ganglionic acetylcholine receptor Ab. It can be tested in Mayo's and Oxford's labs. Mayo is doing it only as a part of some expensive panels, so it is much cheaper in Oxford's Labs.
The test in Mayo lab: http://www.mayomedicallaboratories.c...rpretive/89904
And in Oxford lab: http://www.ouh.nhs.uk/immunology/dia...ntibodies.aspx

Starburst, regarding the question whether to go to a cardiologist or a neurologist, I agree with en_bloc that it is important to inquire about a doctor who specialize in dysatutonomia. It doesn't have to be a neuro or a cardio, I was diagnosed and treated for POTS by internists who specialize in dysautonomia, fainting or blood pressure.

There is a lot of info about dysautonomia in this blog:
http://potsgrrl.blogspot.co.il/
it is mainly about POTS, but most of the tests and treatments for POTS are the same as for orthostatic intolearnce.