1994 BMW 530 Air gets warm when sitting in traffic

In the last few weeks I have noticed my air conditioning is not producing the frigid air it once did. It still gets cold, just not frigid, when driving at speed down the road but when sitting in traffic or at a stop light it begins to get warmer. I looked at it today and when I initially started my car, the AC got cold but as I let it sit there and the temp of the car increased the air from the air conditioner began to warm a bit.