A/C stops blowing cold at traffic lights

[cantabridgian]

Hi Everyone,

This'll be an easy one for you all! The air conditioning in our '85 300CD blows nice and cold when trundling down the interstate, but when we're stopped at a traffic light, the air starts to get warm and humid. Needless to say this poses a problem when trying to use the air conditioning around town - especially this past summer and our current indian summer up here in New England! If I sit in the car, parked, in Park or Neutral, with the air con running, same problem. But if I toe the accelerator and bring'er up to about 1500rpm, the a/c starts blowing nice and cold again. Methinks it's some type of "high-idle" needed for good a/c when stopped that our car currently lacks. What say you? Thanks in advance!!

J