dog snapping at air

Question
I adopted Cujo, a 7 yr. old Shepherd/Rott, a few years ago. For the past 3 months, he has the constant annoying habit of snapping at the air.  He raises his head then moves it from left to right and snaps.  It's as if he is seeing bugs flying by and snaps to catch them. There are no bugs, and he does this is different locations [my house, a friend's house, at the vets, etc.].

His ear infection has been cleared up, his fleas are gone, he isn't taking any meds, he doesn't have cataracts...all issues which I thought could be causing the behavior. The vet is at a loss of what could be the cause. Do you have any ideas about what might be causing this strange, compulsive behavior?  Would boredom and limited exercise manifest itself into such behavior?  The only time he isn't doing this is when he is exercised, his favorite distraction.  Any insight would be appreciated!  Thank you.

Answer
This behavior is known as a stereotypical behavior.  Sometimes, it is caused by ear infection, or boredom, or obsessive compulsive disorder, but it can also be a form of canine epilepsy.  You may want to read this article to see if the behavior and profile fit your dog:
http://www.canine-epilepsy.net/flybite/flybite.html