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The problem is not tinsel remaining in the stomach from a few years back.
The most common cause for chronic diarrhea in cats, which often has some associated vomiting, is inflammatory bowel disease (IBD). This is usually treated with prednisone with or without other drugs including metronidazole. Food allergy is often a component of IBD. Therefore, cats are put on special, limited ingredient, foods to minimize the allergic part of the disease.
Colitis means inflammation of the colon or last part of the large intestines. It is part of IBD.
Intestinal lymphosarcoma is the other major disease that causes diarrhea, vomiting and, weight loss in older cats. Getting a definitive diagnosis for IBD and lymphosarcoma requires intestinal biopsies. These can be done using an endoscope if one is available and it can be done surgically. There is little need to get a specific diagnosis is the treatment for the probable diagnosis is working.
IBD is a forever disease. It can be controlled. It can not be cured. It sounds like you have it under control. Fine tuning treatment involved modifying dosage of drugs. Corticosteroids (prednisone) are used to treat both IBD and lymphosarcoma.
Other immunosupressive drugs sometimes used to treat IBD if prednisone is not effective.
Let me know if you have follow up questions.