It's totally normal to have the condenser pressure fall off at higher vehicle speeds, increased airflow over the condenser will result in lower pressures.

Took my 98 CLK320 out for a quick trip with the diagnostics on. The air in this car is one of the best systems I've ever seen, I've got black leather on the interior and the A/C system will pull it down to a comfortable temp in under 5 minutes from fully baked in the sun with 95F ambient and 80% humidity. Kind of like Georgia weather?

My condenser pressure at idle is 14-16 bar and it falls to 8-9 at highway speed just as your car.

You mention 50's duct temps on the highway, I'm assuming its still hot inside? If the interior temps are near the setting this it's not surprising that the duct temps are not ice-cold correct?

My evaporator temp sensor stays rock steady at 38-39F, what is your evaporator temp? Also check the heater core temps, left and right. Mine are about 44-45F with the A/C in operation. If the heater valve isn't closing properly that could cause intermittent heat when you're expecting A/C.

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