Normally, a small syrinx is an asymptomatic finding on an MRI unless it is associated with Arnold Chiari syndrome. Most radiologists know to look for that association. I am unaware of literature that notes a syrinx that switches between symptomatic and asymptomatic and would be hard pressed to imagine a situation causing that to occur.
You need a consultation with a good neurologist to help understand what is the origin of your symptoms. I can implicate many structures for the cause of your symptoms but with global symptoms. the neurologist is your first stop.
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.