You have probably tried PPI drugs, as they seem to be standard fare for unexplained nausea
cases. If not, you could always try Zantac
, Pepcid, Prilosec, etc. If acid is the issue, these should result in noticeable improvement.
Finally, and I guess this is my most important point. The one thing that has helped me, and made a dramatic improvement in my quality of life is the antidepressant Remeron.