I can't see the cats causing the problem; you could trap a little exhaust heat with clogged cats i guess but if your cooling system is working probably you shouldn't even notice it.
how long has it been since you flushed and changed fluid in your rad?
do both fans run when you are running hot?
could be thermostat, fans, radiator...
...and of course the impeller could be going on the water pump.
95 SE MTX
DH UIM, 65mm TB, SC CAI, Quaife, Centerforce, Fidanza, UR Pulley, F/R Strut Braces, Cross Drilled Rotors, S.S. Brake Hoses, 18" Enkei RS5, SVT Engine Mounts+Inserts, MSDS & y-pipe, Trubendz resonated exhaust & MagnaFlow cat
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