overheat only over 3000 rpm

I am writing here as I hope to find a hint to solve this particular overheating problem:

I got a stock 318 poly engine with the "power pak" option, perfectly tuned and performing. The radiator has brass top and bottom tanks with modern three lawyer cores; it was cleaned by a professional shop ( dismantling the top and bottom tanks and passing a rod in the vertical core passages) to make sure the cores were free and flowing. I use a specific red liquid for radiators. The rad has the proper rubber fittings to avoid the loss of airstream from between its top and the front frame. 13 pounds radiator cap.

So.... It doesn't overheat when I drive around at 1500-2000 rpm, but if I hit the highway, as soon as I get over 3000 rpm it starts to overheat. (the max rpm of the engine is 4400) and if I insist, no matter the four running fans installed on the front of the core and running like hell, (pushing in the right direction) the engine slowly get to the boiling temperature. 230°F.

I imagined that maybe the limestone accumulated in the decades in the inside of the block might act as heat insulator, thereby keeping the liquid from cooling the engine.... therefore I did one hell of a cleaning, making it run with salty water for several days first it (it dissolves the limestone) , then with a mixture of water and baking soda after, (to take the acidity of the salt out of the block), then washing and rewashing tens of times the engine making it run with running water in the radiator (with a hose) while the return hose was detached and discharged the return water in a container.... until tasting the water coming out of the radiator it was clean and almost drinkable. (almost) Then I replaced the water with the proper coolant, and....
the overheating just got a little further up on the rpm. But it is still there. I can't drive at a steady 3500 - 4000 on the highway without overheating.

Any hint?
Please....