Not enough air flow over the condensor could not only reduce the cooling efficiency BUT can also cause the compressor to cut-off because of excessively high pressure on the high side. So either the condensor fan is not operating or it is not operating in the correct direction (i.e. the flow should be from the outside towards the inside). I am not familiar with this car but on my 260E there is a temperature sensor on the engine block which sends signals to the climate control relay, and if this is bad, it could cause the relay to cut-off the compressor. So while you are idling, check the condensor fan and also check if the compressor clutch is actually running when the air blowing is hot. Have you checked the pressures ? What are they 1. when you do get cold air and 2. when you dont ?

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Saumil S. Patel