Sounds like the fan isn't working. Start up, condenser hasn't really heated up yet, but once there it seems like the heat isn't being dissipated during stop and go traffic, unless you are moving to get air through the condenser. Also, could be that the compressor isn't properly compressing unless the rpm's are higher causing it to spin faster....unless it's a variable speed or you are low of refrigerant. In any case, the electrical is easy to rule out, either the fan is working or it's not. Then you'd just need to check the refrigerant in the system, and next move on to the mechanical. Since you have some cooling, it shouldn't be too hard to figure out.

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1990 190E 3.0L