Over heating on the freeway?

Last thing I notice about the fan. It has alot of grill slats.

THink about your attic eve vents. it might cover 12"x 4" (48 sqin) but with the screen in there to keep critters out you are down to maybe 2/3 of that so maybe 32 sqin

Your fan is the same, measure the open space and you might see that you only have 1/2 the sqin of open space where air can flow through. look at the sqin of the radiator and the sqin of the opening for the fan maybe 2/3 of the rad sqin. I think all of this is the problem. loose the shroud and see what happens.