I've always wondered if flex fans influenced this too...the idea that as the engine speed increases @ 55-65 range(as we'd expect w/ 4.10's) and the fan flattens, it actually decreases the amount of air flowing past the radiator....

So then the question becomes, are you also running a flex fan?

fwiw, my LRT does as it should...get a touch hotter in traffic idling, but cools down even in these ungodly temps once I get rolling. AL headed .030 over 408 ci small-block w/ 10.5:1 compression, AL radiator, viscous clutched fan....