@thracian I am a cervical spine surgery patient. What you may want to ask, is if you have spondylolisthesis, or slipping of one vertebra past another. When you have a muscle spasm, it may move these vertebrae around if the discs are not doing a good job of holding the vertebrae in alignment. It seems your symptoms come on when you bend at the lumbar spine and that may be changing alignment if one can slip a little bit. X-rays taken in different spinal positions could illustrate a problem like this.

Jennifer