Ronf42

My 2000 Subaru Outback overheats intermittently. I have had the water pump and timing belt changed. New head gaskets. Flushed radiator and cooling system (radiator is just 5 months old). New hoses, anti-freeze, plugs, air filter. Installed Subaru OEM thermostat. Oil change and filter. New main radiator cooling fan and relay. Hydrocarbon test. New coolant temperature sensor new 13lb radiator cap (from Subaru). New intake manifold gaskets. New crankshaft rear main seals. Increased engine idle speed to 800rpm. Temp gauge still goes almost all the way to the last marker before the red zone, then drops back to about 1/2. Keeps doing this while I am driving. When stopped and idling it shoots ack up to 3/4 or almost all the way. What have I missed/ HELP!!!