What Causes Neck Locking And Stiffness?

Having problems with my neck "locking" in place when looking down at times, pain, stiffness. Had an x-ray Full result: There is multilevel cervical spondylosis most prominently affecting C5-C6. Degenerative anterolisthesis at C3-C4 measures several millimeters. Atlantoaxial and atlanto-occipital relationships are normal. No cervical rib. Lung apicees normal.