Chris you said you had all the above tests but you don't mention if you had the test for Tritrichomonas, it can't be done by your vet and the stool needs special handling as the parasite is fragile outside the body, false negative results are common. I urge you to google Jody Gookin, she has a website that has very comprehensive info on TF. One of the problems with TF cats is that ANY change in diet can trigger an episode. The loose, smelly stools can come and go and many cats resolve it but remain carriers. My two cats have TF but unfortunately can't tolerate the Ronidazole. TF is very common in Bengals as well as other catteries but few vets are knowlegable about it. Please check this out, I wish I had known about it when mine were kittens, maybe I could've saved a lot of money and health problems.