This is what I think. You've got poor decoupling between your sweep supply busses and your video. Add some capacitors to the supply rails and see if it doesn't go away.
A similar condition also occurs in monitors.

Adding capacitors didn't work. I am suspicious that there might be an error in return beam landing, but I can't find anything in the literature (including patents) which allude to this fault other than the work done on image isocons.