The only way to increase the mass-flow of the refrigerant is to increase the RPM of the compressor. That's why the cooling improves when you get the engine RPM up.

It sounds counterintuitive, but you may be overcharged, especially with all the extra airflow on the condenser. Connect your gauges, set the air on max cold, max fan, and leave all the doors open. Have someone hold the engine RPM at ~2000 RPM and watch your gauges.

Ideally you should be ~225PSI on the high side with all that airflow. If you're significantly higher than that, let a couple ounces out and see if things improve. If you're much below, you might be undercharged slightly. Suction side is usually around 25-35PSI with a proper charge.

If the pressures are WAY off, suspect the expansion valve.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

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1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]