I encountered the identical issue with my ALFA 156 V6 Veloce 2.5 Busso. It required some time for me to comprehend it. Under typical driving conditions and loads, it functioned adequately; but, at elevated RPMs and loads, when attempting to circulate the water more rapidly, it overheated until I reduced speed, contrary to expectations given the airflow through the radiator at high speeds. The issue was identified as the water pump. The plastic impeller of the water pump was slipping on the shaft under load, obstructing the water flow. Upon replacing the pump, I was able to manipulate the impeller on the shaft using merely firm finger pressure. It appears to be a prevalent problem, as the new ALFA authentic replacement pump had a metal impeller affixed to the shaft by soldering. While it may not apply to your situation, I felt compelled to mention it because of the same symptoms. Have you attempted to bleed it to ascertain the presence of an airlock? Have you disassembled any components of the cooling system recently?