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That sounds a bit like my parents cat - was rescued from somewhere by the vet and had every test under the sun (things that no-one who had to pay for it would ever do) to try and figure out what the problem is, couldn't find anything. They let us take him with a warning and after a month of boiled chicken we cautiously tried him on normal catfood. Whiskers seemed to be the one that he could deal with but 10 years later I'm pretty sure he eats whatever he can get his paws on!
Nah.
There are some foods she can handle but then she sometimes has diarrhea. Other foods are impossible. Just a bad stomach. Purina for example results in a lot of diarrhea and blood. Other foods it's just sometimes.