This could be a sign of a vestibular problem. You can read about vestibular disease here
www.marvistavet.com/html/body_vestibular_disease.html
It might be due to an inner ear infection that can't be seen by looking at the external ear area. You might ask the vet for a round of antibiotics to clear up a suspected infection and see if that cures the problem.
I think if you read through the above site you will find that could be it. Its a bit similar to Meniere's Disease in humans with the vertigo.
Hope this helps you!
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