Can anyone offer any help in diagnosis of my cat?

About 2 months ago I found out my 13 year old male cat had renal
failure. After taking him to a new vet, the vet realized he had high
protein as well. We did a protien electrophoresis and it was found
that he had a monoclonal ab spike. We did a bone marrow biopsy to rule
out multiple myeloma (from 2 sites) and the biopsy was negative.  So
now the vet said she doesn't know what it is and that we should just
treat his renal failure.

Does anybody have an idea what else it could be and how to diagnosis.
She did another test for Ehrlichia antibody and it was negative.

In summary he has:
elevated bun/creatinine
elevated total serum protein
normal albumin
high globulin
monoclonal spike on electrophoresis
normal abdominal ultrasound
normal bone marrow biopsy
negative Ehrlichia antibody

Please let me know,
Thanks
Virginia