Shaking, twitching cat! What is wrong with her?

My cat is currently around 13 years old and has developed parkinson's like twitching. Its severity changes throughout the day in no particular pattern. She has occasional “spaz” attacks where she jumps up and spins around often with her claws unsheathed, and afterwards walks off like nothing happened. She was a good mouser and although she was allowed indoors whenever she wanted she chose to remain outside. Now, however, she stays inside almost all the time. We have taken her to several vets and none know what it is from, and their answers vary from nerve damage to poison. She is also going to the bathroom and eating quite frequently but is very skinny. We assumed she had worms and was treated as necessary. She is also given Frontline flea and tick drops. Her condition has worsened over a course of about three years. Thank you for your time! My kitty thanks you!

You can check out http://www.vetinfo.com or just click on this: http://www.vetinfo4cats.com/catindex.html#S and then click on T for twitching...hope you find out whats wrong...

Thanks so much for your reply. The website had some very interesting articles but none of them seemed to match my cats problem. Another symptom I forgot to mention was that her hind legs seem to be giving out; she walks in a snake-like fassion, walking foward with her hind end all over the place. Thanks again!