Cat infection

Question
My cat has had an ear infection (or something like that, the vet doesn't really know) for about 6 months now.  
I took her to the vet when it seemed to be getting really bad.  It looked like she had ear mites and she kept scratching at it (her left ear more than her right).  
The vet gave me some medicine to put in her ear twice a day.  I gave her all of the medicine for 2 weeks, and that cleared it up like new.  Then about 2 months later it came back, full force.  
I took her back to the vet and she had me do the medicine thing for 2 weeks again... and it cleared up again.  She said that it is probably some kind of allergy that she has.  I have another cat that hasn't been affected by this at all.  
Then she sent me away with some antiseptic to clean out her ears with once a week.  I have been doing that.  A couple of weeks ago, I noticed that it is coming back again.  

When I saw what your expertise was, I thought that I would see if there is any advice that you can offer me, or if you perhaps know what the problem is.  I am tired of taking her to the vet, spending over $100 per visit and not getting anywhere, it is very frustrating for both me and my cat.  Please let me know if you have any questions, or if I can offer you anymore info.

Thank you!

Answer
This description is very similar to cats we see with food allergies.  The infection and ear disease goes a way with topical treatment, but then it returns when the medicine is stopped.  Ask your vet for a special diet to feed for 2-3 months to see if it goes away with diet.  Sometimes cats will get middle ear infections and polyps.  To diagnose that, your vet will need to anesthetize your cat to get a good deep examination of the eardrum and middle ear.