To me it seems like your thermostat could be broken and it won't close anymore. Now the question is, what was the temperature before it stopped warming up properly? Can you remember if it was higher or lower with the aluminum radiator when driving faster than with the smaller OEM plastic radiator?
When I got my aluminum radiator new a few years ago, the engine temperature was always a bit too high when driving fast (which means about 160 km/h on the German Autobahn). This had always irritated me because the aluminum radiator cools more efficiently. After three years, the head gasket was broken due to the always slightly too high engine temperature. That's when I started to get interested in how the cooling circuit works on my 2014 4/4 with Ford Sigma engine. This is described in detail in this thread.
http://www.talkmorgan.com/ubbthread...the-cooling-water-temperature#Post713083In short, because the thermostat on the Ford Sigma is not located in front of the radiator inlet but behind the radiator outlet in the engine block, the heat of the circulating coolant in the engine block is no longer sufficient to open it when necessary if the thermostat is cooled down too much from the outside.
But all this has nothing to do directly with your problem, but maybe indirectly. If a thermostat always runs too hot, it could be that it breaks down and no longer closes.
