Robert,

I have been fighting the same thing on my 12"GSO RC for years.   I have a truss system.   If you find a remedy, please let me know.  Recently I placed protostar flock on the tube portion of the scope to make it even darker, but I doubt it will help.  It seems to happen when my scope is pointed between 10 degrees and 30 or so degrees, and is less frequent when I am pointed more towards the zenith.  I think it also happens when there is a bright star nearby.