Without A/C manifold gauges it's hard to evaluate your system, but with a little common sense you should be able to find the problem. I have experienced your exact problem when my Aux. Fuse was blown. Replaced the fuse and all has been well since.

If your A/C is cooling when driving but not in heavy traffic, then the Auxiliary Fan is probably the problem. If it were me, I would jumper the connectors at the Auxiliary Fan Temp. Switch at the Receiver Drier (this is the one with the pigtails). This way, the fans are operating when the engine is running and you can test your A/C cooling temperatures while driving in heavy traffic and also test your fans to make sure they are operating. If the Aux. Fan doesn't operate, then check the fuse and the Aux. Fan Relay.

You have received a lot of good advice from fellow members so far.

Good luck,

Dave

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible