Ganzergrey, this is a common problem for many pets in the last 25 years. Which is also the time that pets started developing IBS too and other "diseases of civilization" (diseases that also skyrocketed in humans too). The problem is the fillers the food manufacturers are putting into pet foods, that are not natural for their diet (especially grains). You might want to consider feeding your cat actual food (buying raw fish, raw meat including offal, mice etc). It's going to be more expensive, and the cat would need time to adjust to these new flavors, but if in a month's time your cat stops throwing up, then you'll know what was wrong... Have a look: http://www.youtube.com/watch?v=9ikaFMhBOPs
When I was living in Greece as a kid we had 2-3 cats in our country home, but we were leaving them outside to live their lives freely (even in winter). They would occasionally come to us for petting and food, but they would hunt on their own too, have babies, fight, get killed eventually by other cats/cars... But at least they were not living a life of pussies. They had enough food to eat, enough love from humans around them, and I never seen an unhealthy or fat cat in my neighborhood. While you might think that the relationship between humans and cats were distant in that setting, let me tell you that the only time a cat snack inside our house, was to give birth under *my* bed. She would not have done that if she wouldn't trust me. My mom was pissed off for having to clean up all the blood on the floor, haha (birth took place while I was at school, otherwise I would be the one cleaning up since I allowed this).
Regarding my own problems, I very recently solved my biggest one (only one?), which was a whole galaxy of health problems (over 20 of them!). I was half-dead for 10 years (I was convinced that I wouldn't make it for much longer), and now all I want to do is to live and experience. The Paleo diet brought me back to life, only 2-3 issues remain that will take a bit more time to fix naturally. I really can't complain anymore about anything, it would be ungrateful if I did. Life has been good lately, and I hope others receive the same luck too.