That would help, but likely not fix the OP's particular problem unless the fan or shroud are the issue. The a/c in working condition should function @ idle.
Its very hot and humid here. Same for where the OP lives. I used to have the same issue with my system. Yes it would work at idle, just not near as well. I tried a new fan clutch. That did a little, but very little. Then I tried a 12 blade fan with a new clutch, and that did a little better, but again, nothing like doing the electric fan swap. I did this on my truck, so this is first hand experience. It makes a world of difference, when your at idle for any amount of time in these hot, humid climates. There are a ton of videos on you tube with people saying the same thing I am saying. The large amount of air flow the electric fans pull across the condenser core, is so much more than any engine driven fan can pull, that it is a huge difference at idle. Again, I did this on my truck, and that is why I know what it does. My low and high pressures were with in spec, and my AC did the same thing as the OPs AC. As long as you were driving down the road, all was good, but once you stoped at the red light, the AC went from cold to cool. As soon as you stated moving again, the AC was cold again. Once I dis the electric fan swap, all if this went away. My AC is cold all of the time. Again, this is my experience on my own truck. Im Im not saying that the OP doesnt have other issues, maybe they do, and maybe they dont. And yes the system does need to be in good working order, and pressures with in spec, but an electric fan swap will make a big difference in the way the AC works at idle.