Headgasket? Calling all master techs!?

There is a color changing chemical that you can add to your coolant to check for the presence of exhaust gases. That would be the best place to start, it is possible that your head gasket may be letting exhaust gases vent into your cooling system. Another possibility is that you have an inadequate mixture of water and coolant allowing it to boil, introduce air pockets and overheat as a result. Hope this helps