Gibson has this occasionally. The first time it happened I was terrified (I still hate them) so I rushed to the vet. She told me that some breeds (boxers included) just get unexplained head tremors. It doesn't seem to bother him at all, in fact he gets excited at the peanut butter and extra attention he gets. I have noticed a pattern in that they only seem to happen when he is resting...almost sleeping. When he is active, or even just sitting around hanging out with us, he has never gotten one. My vet said to just watch him and if they get severe or start to bother him, she will explore it further. I know it's hard to watch and it scares you, but it seems to be fairly common, and not something that hurts them.
Hang in there! Jen