The aux fan will not play into this issue. It is there to bring down excessive high side pressure which is obviously not a problem when it is not cooling well in traffic. If it is an air flow problem leading to this issue, then check the engine driven fan clutch.

You should NOT take corrective action until you have completed a proper diagnosis. That is Troubleshooting 101. You need to connect a set of manifold gauges, turn on the a/c wide open, block the throttle to 1500 to 2000 RPM, place a high flow fan in front of the vehicle, let the pressures stabilize for five minutes. You then need to write down the ambient temperature, low side pressure and high side pressure. At that point either post this information here or look up a chart and analyze.

Look before you leap.