What about the heater core?
I've had a similar problem in the past with my '94 V6 Camaro and my '93 Ford Bronco. In the Bronco the heater wouldn't work very well at all and the engine would overheat all the time. I replaced the radiator, hoses, thermostat, switched to electric fans and basically bought a whole new cooling system. After all of that, I decided to replace the $40 heater core and BAM, the heater worked great and the truck never overheated again.
'94 Camaro Z28 - 100K Miles - AOD - 3.23 Gears - Moroso CAI - Trick Flow Elbow - Drilled/Slotted Rotors - Flowmaster Muffler