Extreme chattering.

Hello my senior dog/male neutered/mutt (not sure how old..in the teens) has a history of vascular incidents and over the past few months has very violent shaking of the head where he seems to wanting
to dislodge something from his nose..during bad fits some clear fluid comes out...it could be severe acid reflux when it goes up into his nose and is followed by rapid head shaking and snorting? he does have a loud stomache and belches as well. Then a few days ago he was anxiously and quickly. excitedly picking up some crumbs of a treat where his head was down and toward his body and all of a sudden his jaw went into a rapid spasm...chattering....it looked like a seizure but when I put my hand under his jaw and stood him up it stopped. it continued every time he put his head down so I continued to lift his head and it stopped. the next morning he was licking his legs and it started again so I stood him up and it stopped. Hes on an antacid and keppra since he has had some twitching issues and is a nervous dog which makes his vascular incident risk high ....any ideas on what this is ? also my fears of leaving his alone which of course i must do is that this violent head shaking will break his neck or his spasming of his jaw wont stop since I wont be there to lift him up and he will suffocate or break his neck.

UPDATE from yesterday: Hello, took him in for a neuro consult with the city neurologist and it was said that he is a "risky" in hospital patient as he has had two vascular incidents from stress (once he had his blood drawn and couldnt walk for half an hour after) he shakes uncontrolably and his blood pressure goes off the charts....so he gave me antarobe antibiotic (which he has vomited up) to keep any mouth pain at bay and then talked about pulse dosing anti biotics if it helps. Last night what happened was he was laying down on his side resting then started pawing at his face, mouth and then went into what APPEARED to be a jaw chattering seizure or spasm, head cocked to the left, ..when I lifted him up it stopped and then continued on if he wasnt lifted up for about four minutes. Im terrified to leave him alone which of course i must...
Thank you.