Engine heat soak possibly. A lot of times caused by reduced flow.
Besides the cleanliness inside and out of the radiator, and the screen, a couple of other things to check.
Easy ones -
Radiator outlet hose, make sure it isn't collapsing at full revs when at operating temp.
Fan blades and the belts, many times belts will slip but not squeal over the top of the engine noise, especially at higher revs, as the revs go up the load on the belts increase and they can start slipping.
Check fan ensuring all blades are still there, shroud in place etc.
Radiator cap, get it pressure tested, if it's releasing early, your fluid is boiling at a lower temperature where normally it wouldn't boil. A faulty cap can cause this, as well as a small leak (particularly at the top of your system), it's essential the system is pressure tight to the recommended pressure. So check all ancillary return hoses, oil cooler hoses etc.
Your reserve tank must have fluid in it at all times hot or cold, well above the return tube, also check that tube hasn't fallen off.
Harder ones -
Thermostat, make sure it is opening 100% at the correct setting, a glass kettle or jug is ideal to suspend it in and slowly heat and maintain opening temp (with digital temp gauge/probe) and make sure it opens fully at the correct temperature.
The water pump itself, ensure it hasn't a degraded impeller.
Good luck, hope a cap or hose fixes it!