So many illnesses have the same or similar symptoms, and the range of tests is not always comprehensive enough to enable diagnosis.
The experience of the treating doctor is also a factor.
Do you have a list of all of the symptoms that you have experienced, when they occur and what triggers them?
This would help you and the doctor to see a pattern, if there is one.
Include diet too as there may be food or chemical reactions involved.
You really need to take a very wide overview, and while a daunting task, it may well lead you to find success.

Have a look here for a listing of symptoms for a possibly similar illness to give you some idea of what you may need to think about.

www.hfme.org/themesymptomlist.htm

This is my daughter's website.