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This is tough. The only other thing I may try is an oral benzo like Alprazolam.
The other thing I'd make sure of is the Pred trial. Did you do immuno-suppressive doses 1-2 mg/kg/day for a good 1-2 months and then taper? I would try the higher dose and a slow taper over 6-8 months.
It doesn't sound like true seizures, so that's why I wouldn't necessarily go the Phenobarb or Zonisamide route. I think it's more of a neuro-muscular thing, where benzos would have a better effect.
Of course if they could see a neurologist that would be ideal. I know that's not always in the cards, though.
Let me know how it goes,
Gary Ryder, DVM