When you say negative MRI, is that a brain or spinal MRI or both? I have never had a lesion show up on my brain MRI, it took four years for me to find a doctor who thought it was a good idea to have a spinal MRI. Lo and behold, a big old lesion in my neck causing me all kinds of trouble. Also, be persistent with the doctors. Do not let them stop checking until they have figured out what is wrong with you.
Be joyful in hope, patient in affliction, faithful in prayer (Romans 12:12).