Have you pulled your thermostat and tested it in boiling water?
Perhaps that is the cause.
I had an overheating problem in my Series 80 (non-pressurized system) and I recall that pulling and testing the thermostat was one of the routines through which I cycled before attaining success.
Perhaps if the thermostat functions properly, you need a Lower Temperature thermostat, or a Higher Temperature thermostat.
If you use the Stovetop testing method, be sure that you have a kitchen thermometer in the water with the P-A thermostat to determine the temperature at which it opens.
If you test it in the car, the thermometer will still provide useful data, and more data are helpful.
Again, I believe that if your water-pump runs well and not leaking, your radiator is not leaking and everything else (that you can see) seems fine, then you need to test your thermostat to see if it is either Not-Opening, or Not-Closing.
Good luck and keep us posted.