Kall 123,

What have they xrayed, Wikipedia reference-linkMRI'd etc...?

Lumbar spine may cause this type of symptoms.

Neurological conditions.

Blood flow problems.

*May be start by requestionning...regarding onset, how long it takes to come on, the symptoms they are feeling etc...also go through all red flag type questions again.
*If you can assess blood flow with a doppler or have a GP who can I would do this.
*Check lumbar spine, neuro assessment.

Other things to consider if not already done are femoral torsion, leg length, pelvic alignment...etc. Assess gait including hinging points in lumbar and thoracic spine. It may be biomechanical related.

Good luck..I hope that you get to the bottom of the problem...always ask someone with a fresh pair of eyes to take a look.