Once when I was having a heating issue with a totally different engine (Studebaker V8) if I turned the heater on it would cool down. I think it was just the little bit more cooling that the heater core & fan produced. Not a fix and very uncomfortable to drive in the summer but it let me know I didn't have enough capacity in the cooling system. Also a fan shroud helped. I can not see this as being your problem. It is a puzzler. Is there sufficient air flow through the radiator? Once I had an engine that did not over heat until I put a pusher electric fan on it.
