I had the screen-darkening randomly problem before on my '08 - if you take a flashlight to it, the idrive functions are still working, correct? I had my screen replaced, under warranty, so no idea of cost. No codes either, just the screen. Make sure it's just the screen vs. the whole system.

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr