Probably spurious reflections within the projection optics.
You may want to look inside the TV cabinet to see if there are shiny objects inside that light is reflecting off of. (Careful of high voltages inside).
You can do such things as cover up one or more of the CRT faces inside while someone else watches to see if the ring goes away, this narrows down on where the problem is coming from.
Many set owners line the inside of the cabinet with black cloth to reduce such reflections.
Video hints:
http://members.aol.com/ajaynejr/video.htm
