Did you replace the radiator with the correct one? High temperatures and high humidity in your area can be an iissue if you have the wrong radiator installed. There are at least 3 radiators for this vehicle depending on many factors such as '"tow package" with HD radiator etc. The differences in the radiators are how many rows of cooling tubes it has. It you have installed one "economy" radiator with a minimum amount of cooling tubes, this could be part of the problem. Please research your replacement part before anything else.