Ok, so I wanted to share my results. Just put in the new FSR, and lo and behold, the behavior was the same. However..... I did some experimentation and I found that the temperature setting had an effect on all of this. For instance, when I first tested I had the temp controls set to 68 degrees. Same thing happened. A/C ran for a while and then flow seemed to diminish some with no change in temperature. I then lowered the controls to 60 degrees and flow was restored. Again, as in my initial description of the issue, it sounds like the fan speed stays the same but some flap has opened up, resulting in less volume of airflow... Now I don't know if this was just a coincidence, but I have never noticed this kind of a change before in all of the years owning my X5. Is it normal behavior for the flow to vary some on and off if you have the temp settings set higher than full cold? Is there a flap or valve that switches on and off accordingly? Sorry if this seems like a dumb question but I'm a bit confused by the behavior. Thanks-

EDIT... I should also add that the compressor stays engaged during all of this, with the vent temps essentially staying the same.

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'05 X5 3.0
'06 325i
'90 E30/S52