problem with my car's ac system on 1997 BMW 328i

I drive a 1997 323i, for several months now, whenever switch on my car ac, it works well at first but after a few minutes the intensity begins to drop. It gets to a point where the compressor goes off. what can the problem be.

Asked by for the 1997 BMW 328i
if this car was in my shop, the very thing that I would do
is evacuate the system completely and pull a long vacuum on
the system before recharging. after that, watch the vent temp to see what the results are.