Is your MRI of the brain AND your cervical spine? With contrast? If this was me, that would be my standard. Positional symptoms are an important observation for the Dr to note. A Doctor may even be able to reproduce it in the office or lab. I feel certain you can have a diagnosis and care plan determined. Advocate for yourself without hesitancy bc it's important that you get an intervention for your problem.
Keep us posted. We're with you.