I just replaced a brass radiator with an aluminum one and getting some weird temp ranges. First thing I noticed was that during the first heat cycle the temp would rise to over 210 then drop to about 185, then to about 195. I installed a new 195 thermostat and get the same results, first heat cycle goes from 0-210+ before dropping to about 195.

The temp fluctuates oddly compared to the brass radiator, with the brass it would hit 195 and stay there, with the aluminum it varies a little with driving conditions. Seems to cool ok.

I put an aluminum radiator in a 1937 Dodge pickup I'm building, it has a thermostat controlled electric fan. I noticed the fan didn't run very often but the engine temp was very stable.