Poor ac at idle is not only due too minimal air flow through the condenser, it is also because you are not cranking the compressor at highway speed RPMs. Check to see if auxilary fan is working ok...this should go on as freon temps increase at idle. You also may be a little low on freon.

The overall problem with the AC on these cars is the small evaporator. Even when the system is working optimally, you have marginal cooling/moisture removal capacity simply because the evaporator is relatively small. Just not like the american cars of old that would blow ice within seconds of starting.

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)