For The Past Two Years Around March/April I Develop Severe

For the past two years around March/April I develop severe vertigo. To the point where I can't drive or walk without holding on to something. Last year I was sent by ambulance and had an MRI and CT scan. Nothing showed up in my head/brain. Along with the vertigo, comes random facial numbness and some slurred speech at times. Both episodes, I was prescribed Prednisone and Valium. Last year it seemed to do the trick for the most part. Currently, I just finished my last dose of prednisone. I have only felt minimal relief. I also saw a neurologist, not even he could explain what's going on as my brain images do not show abnormalities. Next I'm thinking going to an ENT. But, I would like an opinion from someone as my PCP says she thinks it's just from my allergies in the spring. To be honest this seems more serious as I have multiple symptoms that are worrisome and life altering.