Instability at C1-2 occurs from a tear of the transverse ligament which holds the odontoid process of C2 against the front of the ring of C1. Pain can still occur if that ligament is intact from degeneration of the facets of C1-2 just like arthritis can occur at the hip or knee joints. A degenerative joint would not however cause instability but decreased motion at the site. If your C2 “slips forward and backwards” on the DMX- and there is more than 3.5mm of slip, an MRI of the transverse ligament is recommended to make sure this structure is not injured.
DMX or fluoroscopic X-rays are generally not recommended with diagnosing instability as the dose exposure to X-ray is significant and the information can be gained by standard positional X-rays with much less X-ray exposure.
Your symptoms appear to originate from multiple structures. Hopefully you have had a good evaluation by a neurologist.
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.