Your description reminds me of when my immune system was totally over-taxed with allergies; I'm allergic to alfalfa (sp?) and hay. I had white spots throughout my mouth. The nodes were all inflamed.

An ENT doc, an Internal Med. doc., etc., might fare better in getting to the root of the prob.

P.S.: I just re-read your post and see you stated the spots have come and gone; with my allergy-related spots, they only increased but didn't go away until after heavy doses of allergy med.

Dry mouth, it made me think of Sjögren's Syndrome... or possibly a form of shingles, which is in the herpes zooster "family."