Cat itching and pulling out hair.

So for a while now my cat has pulling out the hair in her back. You can see her back and tail area spasm as if it is really itchy. She will lick it for a bit than starts to pull her hair out. She now has 2 bald spots on her back. At first I thought it was fleas but we have another cat in the house and she is not showing any signs of itching.  I have taken her to the vet and she said it could be food allergies we changed up her food a few months ago but still she itches. I told my vet but the only thing she does is give her shots of cortisone. My vet thinks that it also could be related to stress since the 2 cats in the house don't really get along. I have done all I can to reduce this stress by providing spots where they can hang out away from each other. This has drastically reduced the amount of "fights" they have but still she itches. The other cat is going away in about 3 months since her owner is moving away. My vet says to just wait and see if that will solve the problem but I don't want to wait that long. She is obviously very uncomfortable and it kills me to see her itch and pull her hair out.

I have no idea what to do.

When you mentioned that she appears to be "spasming", I immediately thought FHS, which is discussed in this thread:  http://www.thecatsite.com/t/283070/feline-hyperesythesia-syndrome-twitch-skin-syndrome.  If you read some of the links in that thread, you will see some references to calming agents, which may help no matter WHAT the cause.  There are also some good references regarding food changes too. 

Thank you so much! My cat pepper is a Siamese mix and it looks like this a problem for this breed.