This could indicate a nerve issue. There is a nerve that runs around the head from the back of the neck and if this is 'pinched' perhaps by a disc issue that can cause problems all around the head down to the jaw. A more serious pinch can cause loss of muscle use in that area.
Choke collars or dogs tied out that hit the end of the tieout frequently and hard can lead to such an injury and head halters when used on a long line may also cause a problem by twisting the neck.
Another possible issue is with the jaw hinge or a dental problem such as a broken tooth leading to the quivering.
Facial tremors can also be due to a mild form of epilepsy where the dog stays conciosu but has a twitch.
You can check around in the mouth to see if you can spot a problem. You can try a harness for walks (they make no pull harnesses as well as regular ones) too but I'd suggest you consider seeing your vet about this. If the dog doesn't do it all the time you might try getting a video, if you can, of the problem so your vet can be sure to see what you are seeing.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics