There is a product called Dr. Elsey's Cat Attract that you can add to the litter, have you ever tried it? It does sound like this may be a stress related issue though. I'm not usually one to recommend drugs but there are some available which you may want to ask the vet about. Good luck at the vet and let us know if they find anything.
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler