It could still be a seizure. We had a Boxer who seized with no physical
signs of distress. The only thing we ever found was that he wouldn't move.
He'd snarl, follow you with his eyes and move his head but he didn't leave
his position otherwise. Seizures can take many forms.

Also, Boxers (don't know about PBs) tend to suffer brain tumors so this
might be a cause as well. Definitely something to get him seen by a vet
for. Unfortunately only a costly procedure would show if its a brain
problem otherwise epilepsy would be a guess. Trying him on Phenobarb or a
mixture to see if the shaking stops would probably be the best way to either
confirm or rule out epilepsy.

--
Tara