Just a note: Still happening with the latest linux-image-4.9.0-3-amd64 (4.9.25-1). Still can't coax out any more detailed diagnostics. Fortunately the Jessie kernel (3.16-0-4-amd64 3.16.43-2) remains rock-solid and the Stretch userspace doesn't have any apparent issues running on it, so I am using that as a workaround.