I was going to say fat mid range, but you have tried two carbs. I had a fuel injector that was super fat in the mid throttle settings and caused similar grief. Will it flash when not fully up to temp? I am thinking of a test when it is as cold as you can without hurting parts. That might give you some info. You could also go way tight on plug gap for a test, to see if the ignition is going away (.020?)