If this were my issue I would get out the infrared temperature gun and find where the overly hot or overly cool areas are. A few years ago I had my '68 RR overheat and puke coolant only when I parked it and it heat soaked. Thought the 26" radiator was good and could see good coolant flow looking in the cap opening, had a new water pump and thermostat, good fan shroud, the right pulleys and everything in order. Pulled into my good friend's driveway and sat there for a few minutes and started puking coolant. He went to his toolbox and pulled out the infrared thermometer and the lower 2/3rds of the passenger's side of the radiator was stone cold as it was plugged. I couldn't believe it. Re-cored it and no issues even with a cammed engine and A/C. Get the infrared temperature gun out and start checking where your issue might be.