I'm trying to remember if we ever noticed anything similar with Evan when he was on pred. Neither hubby nor I can recall anything like that.
It's just her lower jaw? Not her whole head? Almost sounds like a mild seizure in that she stops when you pet her, but I'm thinking a seizure would involve the whole head, not just the jaw. Can you video an episode to show the vet?
Maybe someone else has a better idea of what's going on