I had a similar thing on mine a few years ago and I think others have too, the tiny fins on the radiator corrode and fall off making it more difficult for the radiator to cool the water when at high revs, might be worth checking this out before you start worrying about a head gasket fault.
If the rad has been pressure washed a lot this can break the tiny fins off too, also it could have dried mud in there if you have been laning or off-roading somewhere.
It could also be a thermostat problem if its not opening properly.
Hope this helps.