Randi, my mom's 13-year-old cat just started the same thing - lethargic, not eating or drinking much, puking what he DID eat, and all the blood and urine tests were normal!

KAK mentioned that her RB Storm went through the same thing and had small intestinal disease. Some prednisone really helped him. You might want to go through Shiney's thread and see if anything there helps.