Just a thought and I hope a more experienced AC person replies. There is a difference between driving at high speed and driving at slower speeds with the engine wound up in a lower gear. The ram air effect at high speeds would be very much greater than the air flow at slower speeds. Since, I believe, your car has an electric fan, the air flow is determined by the car speed, not the engine speed. This air flow over the AC condensor may trigger the AC cut-off, perhaps an icing condition.

This may be the dreaded 900 vacuum problem in the heater/AC vents, not a compressor outage. Try your 50MPH+ test again, but when the dash vent airflow dies, feel down by your feet where the airflow comes out when you select the "Floor" setting. On my 940, this is where the cold air detours when you use a lot of throttle or climb a hill. Both of these driving conditions yield low manifold vacuum. If you back off the throttle, even at high speed, does the cold air come back to the dash vents? If so, look up the 700/900FAQ for considerable discussion and a number of suggested cures for this rather common problem.
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Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD)