compressor rpm's are key to cooling loss at idle

[wols0003]

I live in Palm Springs. It's very hot here during the summer; 105 to 118 everyday. Some interesting things that I have noticed about the ac system in my 83 300sd with new compressor, new expvalve, new mb condenser and r12. During the afternoon heat, if I get stuck at a red light and idle for more than 30 seconds I lose most of the system cooling. However if I ad some rpms up to about 1300, my cooling remains nice and cold. Also, when I drive through large mall parking lots at low speeds (5mph) if I drive in 3rd or 4th gear I lose most of the system cooling. However, if I drop it down to first gear, I get ice cold air, just like driving at speed. It seems to me that compresser speed-engine speed is the critical factor in system cooling (at least with r12) as opposed to condenser capacity-air flow.