If you are idling, the condenser will not help you beyond the limitations of the system design on a hot day. You can blow all the air you want across that coil, but if enough refrigerant isn't moving through it, the system capacity is greatly reduced. Remember that the compressors in these cars are FIXED CAPACITY. They're designed to produce the correct amount of refrigerant flow when the car is cruising. If the RPM is too high and the pressure in the system climbs to a point that is outside the design limits of the system (such as when accelerating or passing), the pressure switch will cut the compressor off.

The converse is true when sitting at low engine RPMs. The expansion valve will attempt to keep the pressures elevated to maintain cooling, but it has to throttle down to do this. Your system capacity is reduced, and as a result your car heats up in hot weather.

If the A/C blows frosty when you're moving, cools the car off quickly (5 mins or so in the dead of summer) when it's been sitting out and sun-baking, and the temp rapidly drops when you get the RPM up around 1000 RPM or higher, the system is probably just at its capacity.

From everything I've read, R152A is about as close as you're going to get to R12 performance. Make sure you charge the system correctly if you're using it, you use just a bit over half the weight of R12. If your car has a "recirculate" button, use it judiciously in hot weather, it makes a big difference. My SDL will get so cold with the recirculate button pressed that you wind up adjusting the temp wheel. Of course, I haven't dealt with 106˚ temps yet. I have driven it when it was 95˚ a couple weeks ago and it did fine.

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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!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]