99 Subaru Outback overheating

My car overheated quite badlly a year ago. I had driven a long way, high speed, with
AC on, and then let it idle in a parking lot for 10+ minutes. Very hot day, it was a recipe
for disaster. I had to have it towed back home. My mechanic found that the radiator was
full of rust and sludge and he replaced it. However, the car has seemed to run hotter
ever since. I should disclose that I live in Florida and the conditions are always hot.
Now, a year later, my car temp rises when I am driving, with AC on, in the afternoons.
Especially if I am at a traffic light or idling too long in the school pick-up line. I see that
there is steam coming from a radiator in between the main radiator and the front grille.
What is that part? It is rusty and I can see the steam coming from one corner. Is there
some sort of compound radiator system?