Perhaps your cat is just sensitive to one of the ingredients in the food. Does your SD food have corn listed high on the ingredients? If so, you may want to consider switching to a meat-based food like Canidae or Nutro, etc. Just read the labels. Or, like Jeannie said, switch (at least partially) to a good canned food. Someone here posted a good link about how canned food may actually be better for cats anyway, which is against what we've all been hearing for years now.