Aggie, I just wanted to let you know that we did have success by isolating a cat. In our case the problem was the "new" kitten (a year later). It sounds like you only have the one cat, so it's unlikely to be issues with other pets.

I found a lot of good advice on http://www.catforum.com. I didn't log on or sign up, I just read some of the relevant threads. They also said that a LOT of cats end up at pounds because of litter box problems. I easily found our cat's story over and over.

Good luck, hope your kitty's behavior improves. It can be so frustrating.