Fab insight Reg! Hope the OP responds.
I'm not a vet but from what you have written this sounds neurological not behavioural. Prozac sounds like the wrong call (a band aid solution) for a neurological problem or a problem that this vet cannot figure out.
If budget is not an issue I'd recommend seeing a veterinary neurologist.
Some people love their current vets - others don't. If this were my pet I might well consider a second opinion from another vet if I didn't have full confidence in him/her.