This is a common problem with the X5, I own an 01 X5 and I recently just hit 117,609K.

The down shifting became such an annoying problem to the point that I try not to stop the car as much while i drive, i tap on the brakes every now and then just so i don't feel it as much.

Im very grateful I have 3rd party warranty on my X. I will be rebuilding my transmission i figure around January, dont ask me why I am waiting.... Its for a few reasons...

I noticed alot of X5s past 100K have a hard down shift from the transmission, I also have had a few friends who rebuilt there transmissions and it was gone and everything is fine with there trannys now. I know someone with over 218K now and hes running strong after a rebuilt transmission. He of course had to maintain it properly by changing Spark plugs on time, replacing brakes and rotors on time, do your oil changes on time, not KILL the car by driving constantly on 7 RPMS....

From my personal experience, as long as the engine has been taken care of, they can last a life time....