Yes check all the basics first like making sure all parts are functioning correctly. I have seen quite a few of those aftermarket clutch fans not getting full engagement in my time but otherwise look fine. Did you set the gap on the waterpump divider plate with your favorite BFH? I see from your response below that you have a/c an important thing to know when diagnosing a hi temp problem. I not sure if you are aware but the factory had a few tricks up there sleeves when it came to keeping our a/c birds cool. One was pulley sizes. Here is the factory manual showing the part #s and the pulley diameters are all the way to the right, make sure you have the right ones. http://www.teufert.net/partbook/67-76/1-i.pdf . Is the lower baffle in place? #3 in this diagram http://www.teufert.net/partbook/67-76/8-i.pdf Although more for cooling at speeds it does block off the holes underneath so that air only comes in mostly through the grills and the spaces below in front when sitting still and once underway does not allow the air that is ram fed through the grills to spill out below. And how about all the masticated filler pieces that seal up all the gaps around the radiator? look in the service manual under cooling system and they are listed as seals in the diagram for A/C cars https://www.firebird400ho.com/mdocuments-library/ Good luck and let us know what you find.