Doug,
Have you checked your flats ?
Stack 5 copies of the same flat together on their own and see what you get.
I am just wondering if its an additive problem of say out of focus dust motes being diffracted/affected by the 400mm aperture blades ?
Try some different apertures and see if it changes.
Otherwise Octane is on to it.
Just guessing really
Of course the Worminids were passing through that night you know, with their radiant being in Orion
Rally