I also have a 2002 that tends to shift at random around 50-60 mph,, it will go down to 4 or 3 like it is trying to kick down to passing gear but only during very light acceleration. I also have the down shift problem. It really doesn't appear to be in the transmission, because it doesn't slip or not shift correctly. I had the exact same problem on a Jeep and it was the Throttle position sensor making the computer think the throttle was at a different location and shifting the trans according. The X5 doesn't have this sensor but rather an ECU which is very expensive. It appears to be electornic and not mechanical, hence the reason many people rebuild the trans and it still does the same thing. Maybe a good BMW trans man could shed some light...