this system is equipped with a pressure switch which cuts off the circuit of the ac compressor when the system is under-charged or over-charged. if i will make a guess, the system is over-charged. why did i say so? your ac gets cold then quits after a while which makes me think that the freon gas expands when it gets hot, therefore, expanded gas will build up high pressure and cuts the ac compressor circuit off. check your refrigerant pressure try to relieve the pressure a little bit if you think its too high.

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'85 300D Turbo - CA Version