Sabertooth

I've got an overheating problem. New radiator, water pump, thermostat, head gasket, coolant cap.
The car runs and drives fine but over heats after 20min or so of driving. There's no one thing that I can do to get it to overheat hills, high revs etc. I've used the schwaben coolant vacuum fill tool to fill the coolant/check for leaks and also used the bleeder hole in the heater core hose. When the car over heats I can un screw the coolant cap and the temp will go back to normal almost instantly
I'm at a loss on this one.........