We switched to Nutro line of cat food. Cleared Skinny's diarreah up within a week if my memory serves me. All I know is 2 years later no diarreah
I've introduced good quality can food (PetSmarts brand and Nutro brand) without problems.

Skinny had MAJOR coccidea problems for 2 months. We switched vets after 2 months, our new vet (and current one) went the change in food route because he felt the antibotics which we were on for over 2 months were useless at that point. He was correct.

I would seriously look into good quality food minus all that byproduct/corn crap and see if that doesnt help.