New overheating problem

[Red]

Only two possibles, Ivan.

You know the coolant is being lost after everything is thoroughly heated up. So...if you are lucky it is just the radiator cap. If you have the same crappy anti-leak antifreeze that most of the US is floded with, there can be scale under the cap, so remove it, clean thoroughly, make sure it is seating well. Any radiator shop can test the cap because sometimes they fail and let coolant out--in which case your overflow bottle (you do still have it, yes?) will be too full when you pull over while the car is still hot. (It will be sucked back in as soon as the car starts to cool, don't wait.)

If the cap is more than five years old, it pays to replace it anyway. Make sure the new one is the same pressure rating as stock.

If it isn't the radiator cap...that means something on the engine is heated up and warping or opening up. Usually that would be a loose head or bad headgasket. It could be a bad water pump housing. Easiest way to check that is to clean the engine (steam clean if you can), let it dry, and then dust the likely places with talcum baby. Not "baby powder" made with cornstarch, but real talcum powder aka talc.

The talc is white and easy to see. After you make another run...if anything was leaking it would leave trails where it leaked, also easy to see. No need to wash off the excess talc unless you want to-- it helps protect rubber parts and there's nothing that eats it, etc.

If there are no outside leaks...that leaves the headgasket. Exhaust and compression checks, or sending out an oil test sample to see if there is excess coolant in the oil, might turn that up.

Did you do any work on the head on this engine? And if so, torque it and then retorque it after it settled in?