Cat has had diarrhea for 9 months?

Hello! My cat Kirby has has diarrhea now for nine months. She is eleven months old, will be one year old in September. Let me give some background info.... I got her from a breeder in December and she had diarrhea from the moment I got her. I asked the breeder about it, and she did not realize anything and none of her siblings had/have diarrhea. I take Kirby to the vet where they test her for giardia and the answer I got was very strange. They said she tested positive but could not find any of the actual parasites in her fecal sample. They gave me metronidazole and all was well until about a week later. Yep, more diarrhea. I take her to the vet again about two weeks later, they give more metronidazole, and send me on my way. During this time, I tried giving her purina one in December, blue buffalo in January, Orijen in February, Royal canin and science diet in March (she would not touch either), natures variety in April, and hills prescription food in May then stuck with Canidae since it was the best at giving her less extreme diarrhea out of all of the others. She was given another round of metro in April, flagyl, and Albon in May. They all help for maybe a week and then it's back. She has gotten three fecal tests since the giardia thing and all have been negative. She has been tested for the common stuff twice, and the third was for trichomoniasis and nothing. The vet has given up and wants to hypoallergenic food. I decided in May to stick with Canidae and try benebac for a month. It didn't do anything sadly. I tried pumpkin and nothing. June I tried something called Kochi Free, and it didn't do anything either. I was trying Metamucil this week and it seemed to make it worse last night. I am going to try a new vet on Friday but I really need some ideas as to what this can be. She has no other symptoms. Just diarrhea. Please help, I have to chase her around the house twice every night to clean her bum or else she gets her leaky bum on everything. really ANY info or ideas can help. Thanks.

Has your Vet done a pct test on her?  It's a more detail test that has to be sent to an outside lab where they test for things like T-Foetus.  It could be that, OR it could be all the changes in food.  Food changes can easily cause intestinal upset. 

I would first start with the PCR test and then look at a hypoallergic food if the PCR comes back clean.

The fecal tests that vets do in-house very often miss parasites that are still there nonetheless, and a PCR test that is sent off to a separate lab is the only way to be sure that a microbe is not present. There are many different strains of coccidia, giardia, and other common intestinal parasites, and some of them are more resistant to medication than others, which might be why the diarrhea keeps coming back. If it is a parasite, then it also makes sense that Metamucil would make it worse, because these parasites do especially well if the cat is eating a diet high in carbohydrates and fiber.

One treatment that you might want to look into is trying to S. boulardii probiotics. Some catsite users have had success treating a few different parasites with it, because it crowds out the bad microbes and allows healthy bacteria to thrive. You can read more about it in this thread: http://www.thecatsite.com/t/283161/saccharomyces-boulardii-use-for-diarrhea-and-gi-disease-incl-ibd