I had a similar problem with mine, air kept getting into the system and collecting in the radiator. I fitted a restrictor in the small hose to the top of the expansion tank and the problem stopped. It was an item that Caterham used to fit to the Vauxhall race cars, no longer available now though. All you need is a piece of round ali or plastic just a bit bigger than the internal diameter of the pipe (about 8mm I think should do), 10mm long and with a 1mm hole through it. It needs to be a tight fit in the pipe. It restricts the flow enough to stop aeration in the header tank that then gets drawn through the rest of the system and collects in the radiator.
Worth a try before going to any more serious actions.