As we age the gel like material in the back of the eye "formed vitreous" liquifies and the vitreous body "shakes" when the eye moves. That is not abnormal. If you have had two exams by a competent ophthalmologist then things must be okay and you should accept the shaking unless something changes.
Call your eye MD and ask for some reassurances.
JCH MD
I have my eye checked twice and they said the back of my eyes are shaking and my eyes are changing they have checked my retnia twice with dilation. However what's shaking in the back of my eyes ? And I seen my right eye shake in the front like it's viberting.. And it Kept blinking by it self for 1 minute in my right eye.
My recommendation would be to arrange for a retina specilist to repeat the eye exam with dilation of the pupils. Find one near you at www.aao.org
JCH MD