Oscillating objects. Reason for this?

When I stare at an object, be it anything, and concentrate, I see it moving. It sometimes feel like vibrating and sometimes just shaking or even oscillating. The rate of this movement sometimes get amplified to quite an extent that it causes nausea and discomfort and sometimes even headache. I find it very disturbing. I first noticed it 11 years back and consulted a doctor who referred me to a neurologist. But without even conducting any checkups, it was concluded that it was due to stress. I started noticing the symptoms again 2 years back briefly. Now I experience the same quite often and feels like it exist almost everytime with the rate of disturbance mostly mild. What could be the reason for this?