Q: Why does my air conditioner stop cooling when the car is on idle or at a stop in a traffic light?

Hello. Was the A/C working properly before you had the condenser replaced? Your previous mechanic was correct that the condenser was likely the source of the issues. Now that the condenser has been replaced, we know its working and can tell its the cooling fan that is failing to provide adequate air flow. You should turn your attention to the fans. They can fail in several ways including the fan clutch, fuse, and its main control module. You may also want to verify that the connections are clean, dry, and tight. The first step is to check and see if the fans are working at all, or if they have not been working this whole time. If the fan is not working at all, it or its motor should be replaced. If the fan is working intermittently then there will need to be more testing to determine what is causing it to only respond to certain signals.