What Causes Vertigo With All The Tests Normal?

Dear Doctor, for the past one year I have been suffering from vertigo (it has occured thrice in the past year, with different symptoms. First time it was with some ringing sound in my right ear, second time I had some nausea and the third time was recurrent and no other side effect. MRI done for the inner ear, spine and brain is normal. Doppler scan to check blood vessel blockage to brain is also normal. Now I am on Vertin 24mg, Bilovas and Diligan 25mg. Still there is no cure. what could be my problem and when will I become OK