Rick
If your radiator has a standard top air bleed line back to the overflow tank via the fixed under car bleed tube , check the undercar bleed tube is not blocked and actually is venting properly .
I just replaced the entire under car bleed system as the steel bleed tube was blocked virtually the entire length with a solid sludge buildup. I replaced it with a 6mm 316 SS seamless tube. I cut it up when it was out and except for the very ends it was blocked all the way.
It easy to think something is working just because its there....and if this bleed line is blocked it can be quite difficult to ensure all of the air is actually out of the top of the radiator and overall system.
Once the coolant in the RHS radiator tank has an air bubble in it , because of the location of the fan thermostat bulb at the top of the RHS tank , the presence of air can affect the thermostatic fans from cycling correctly , as the temperature bulb in the radiator is contacting low conductivity entrapped air not coolant fluid. This rapidly results in a cycle of overheating events as the fans don't activate. More boiling , bigger coolant loss , larger air bubble , and around it goes.
Hence symptoms like you are describing.