1996 Pontiac Firebird over heating with A/C on in traffic

Flease help. I live in florida and cant use my a/c.
In stop and go traffic, with my a/c on, my temp. Gauge will go into the red. About 240 degree's.
This is the only time it will overheat.
I can drive all day on interstate with the a/c on and never overheat.
And of course, with no a/c on, I can drive all day and the temp. Gauge never hits 180 degrees.
I already replaced the water pump, the radiator, 2 fan motors, and thermostate.