If it will run cool at highway speeds with the AC on but overheat in traffic you need a better fan (assuming you have the correct pump and a stock pulley). What fan do you have now? If it is stock try a new fan clutch or a flex fan. Do you have the fan shroud on? If it all checks out you may need a bigger radiater.
One more thing... using a thermostat that is to cool can cause a car to run hotter sometimes 
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79 Hatch: NA 347, Stage 2 Canfield Heads, Victor JR. Intake, Blueprinted Holley 750 HP, Solid Roller Cam, Shooting for 10's on motor
93 LX: Tremec, dual friction clutch, fms flywheel, Hedman shorties, O/R H-pipe
Ricer hater's club member #49
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