Grandprix Gt 02

So I have been fighting with this car ever since I got it. Engine is 3800 series 2. Engine code p0300. The problem is my cat keeps getting clogged. I've replaced upper and lower intake manifold gaskets, cleaned throttle, new throttle gasket, cleaned injectors, new injector o rings, new thermastat, and gaskets. New air filter, new plugs and wires, new coils, new icm. Cleaned maf, new fuel filter, new coolant elbows, and a new universal cat that was only 3 months ago and is now clogged. My last cat was clogged entirely nothing but black sludge. So I fixed all the coolant leaks and added new cat and the car was running brand new now it's showing symptoms of being clogged again. It runs real cool on the highway but when I get into the city with stop and go traffic it overheats with the Guage fluctuating and performance drops. Maybe my fan isn't cooling my engine? Idk. Any advice would be appreciative.