Boxer and headshaking
We have recently noticed the same problem with our white Boxer. She's a couple years old, and has never done this before - till the other day.
Its only her head that shakes, and she is still aware, walks around, sits on command, etc. - as if nothing is wrong and has no idea her head is trembling so. It lasts maybe a couple minutes and then goes away. She is on no medication, has had no recent shots, nor has she been on any flea preventative. Also, she is primarily an inside dog.
It happened once upon waking in the morning, again the next day... and a few days later (today) around noon.
We had a mastiff not too long ago diagnosed with diabetes and who had occasional seizures, so we're looking into the possibilities of either.