It most likely a combination of running lean, hot and the ignition timing being off. check intake gasket,have the injectors tested for flow or do a pressure drop test after assembly make sure the ignition timing is right check the cooling jackets for obstructions ( might be a good idea to knock all the freeze plugs out and check around the outside of the cylinder walls for gunk same goes for the cyl head). Once all together check all sensor that are related to the temperature and fuel system feedback system to make sure that all of those are functioning correctly. Lastly when all together check operating temperature with a infrared temperature tester or something compareble. If all that is well you should have no more problem
