A 35 year old woman comes with a long history of neck pain and back pain for the past few years which has become more troublesome in the past few years. She says her pain radiates throughout the back to her buttocks, sometimes leaving her numb. She gets cramps in the legs while standing or while walking almost suddenly and is relieved by sitting down or by flexing her hips. She complains of normal appetite and says that her sleep is good as before and undisturbed. Her leg raising test is not positive and there is no spinal tenderness or deformity. The most probable diagnosis is