It's sounds like your evaporator is freezing up. There a lot of threads on this already. Turning the A/C off will let the ice thaw out and then using a fairly high fan speed and re-circ will help to keep it from happening.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)