Could be the food. If you are feeding dry Purina, I understand that will cause bladder problems. Here's some info from
http://www.soyouwanna.com/site/syws/traincat/traincatFULL.html
"Here are some methods for correcting accidents:
Cats never poop where they eat, so put her food bowl near wherever she had the accident.
Like most of us (with the possible exception of frat boys), cats like to use the facilities in private, away from loud noises or other interference such as dogs and toddlers. If the cat is going in hidden places like under the bed, this could mean she feels too vulnerable in her box. Getting a cover from the pet store or making one from a cardboard box may lure the cat back to her original litter box. Put the box where the cat can reach it easily.
If you have more than one cat, determine if one is preventing the other from getting to the box if they don't get along. If this is the case, then get two litter boxes. Then one will always be available.
Consider if the cat has had an unpleasant experience there, such as being grabbed in mid-poop and whisked away to be neutered. In that case, simply pick a different location to put the litter box. Cats are smart, but they ain't that smart.
Confine the cat to a small area, like one room instead of the whole house. That way, Fluffy is never far from her litter box and has a lot of time to think about getting familiar with it again. Once the cat is using the box again, slowly let her have access to the rest of the house.
Make sure you thoroughly clean wherever the cat had the accident. This is necessary not only because you don't want your house smelling like cat pee, but also because the lingering smell of her feces or urine will serve to jog her memory later that this is a toilet area. So get rid of the smell. (Note: don't clean with ammonia, as that will enhance the scent of urine.)
If you need to change the location of the litter box from one place to another, one technique is to move it a couple of feet at a time, with a few days in between, so the cat gradually gets used to it being elsewhere. If the cat stops using the box, this is your clue to move it more slowly.
For the truly ambitious cat owner, there is a way to train a cat to use a toilet, thus eliminating any need for a litter box. We're serious. People actually train their cats to use a people toilet, and the cats don't seem to mind. We're not going to get into that method of toilet-training right now, because we just ate. But to read a detailed account of a success story, visit
www.rainfrog.com/mishacat/toilet.shtml. This site also has some entertaining photos."
Good Luck