hi,divedoc. Whatever is happening after ten minutes of activity is not happening while static and supine in the MRI or while stationary with the EMG or the Nerve conduction flow.
A chiropractor will give you the best physical exam for motion mechanics. There could be restrictive flow during activity. It rather sounds like central canal stenosis. It could be vertebral slippage spondylothesis aggravated by particular movements. https://www.cedars-sinai.org/health-library/diseases-and-conditions/s/spondylolisthesis.html