I know your vet checked her ears, but did she consider perhaps a middle ear infection? I don't think this shows itself as infection in the ear as such - so there wouldnt be anything obvious to see, but it affects balance. Its just that years ago my cat Cassie seemed disoriented, not walking straight, wobbling when walking, shaking her head, and my vet thought maybe middle ear infection - she had a long course of antiboiotics and was fine after, no further probs. Just a possibility maybe.