I agree with Biff--it may be the flavoring in your ejuice. If you're
vaping strong flavors, try switching to clear, clean, barely flavored eliquds or even try unflavored for a while and see if that helps. Don't forget to drink plenty of water--vaping is dehydrating. You may also try some Biotene products--they work wonders--mouthwash, gum, toothpaste, sugarfree lozenges--they all help.
Biotene Dry Mouth Products
Good luck and happy vaping.