Are you of any particular racial/ethnic background? Some forms of some diseases are prevalent in only certain races/ethnicities; you might try to find a neurologist or immunologist that's the same race/ethnicity as you, or one that specializes in your race/ethnicity. Sometimes they will know about things that aren't even on the radar of people looking at the general population.

Also, there are some TV shows about "Mystery Diagnosis" stuff; maybe you can get yourself on one?

Good luck!