i had this same problem with my 325ci i would turn on the ac and only would work after a few miles on the car and at driving speed. then i had it checked and they pulled all the old stuff out and added the ammount it needed . it worked for a few hours and the prv went off blowing all the new freon out. and i was back to the same non working ac. after some looking into it we found that the ac clutch was bad and it was not cooling it down and the press got to high in the ac and pop the prv. so we changed the clutch and it has worked fine ever sence. so i dont know if this will be the same for the x5 but it might help in your quest to fix the ac

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2003 4.4i black on black