Story time, my stocker R31 (RB30E, clutch fan with straight blades) used to get hot sitting in traffic in summer, had to keep the revs around 3,000rpm to keep it from flaming up (would hear the fan moving a lot of air).
Issue was a tiny crack in the factory plastic end tanks + lazy clutch fan hub (hence the revving while standing, changed fan to an R32 one, much quieter). Haven't had an issue since.
OP mentioned he put a locked hub and temps were fine. I'd question this "brand new" hub.