The radiologist probably missed the chiari malformation the second time, it should also be correlated clinically where you could have posterior headaches, dizziness and ataxia,
fainting with a cough, and weakness or numbness, otological manifestations like aural fullness, tinnitus, vertigo, fluctuating hearing. Any investigation, MRI in this case needs to be used in coherence with a clinical diagnosis, a neurologist in your case. Take care.