Lots of info about your issue. As I said; I believe the consensus is that most have
found a way to live with it. It's just the nature of the beast. I know this is not
what you want to hear. I actually found a way to avoid the downshift issue by
knowing the symptoms and avoiding the conditions that make it happen.It only
happened once this year and that's because I was talking to the CEO instead
of paying attention.

http://www.xoutpost.com/bmw-sav-foru...ownshifts.html

I know it's not a big deal and it would definitely be easy to deal with. BUT, my concern is it's a sign of a failing transmission. I don't want my CPO to expire in February and then in April my tranny goes completely bad and then I have to pay 8k+ for a new one.

Additionally, the fact that I drove another X5, same year, same engine, and more miles - and that car didn't have the issue.. is another thing that leads me to believe this is a problem with my transmission.

Most likely it is. Lets assume it 100% is an issue with the copy of the tranny put into your car. Might not be terminal, might just be an annoyance... thats a whole nuther topic

But how do you prove it?

One thought- a bit esoteric...how about pulling a sample of the ATF and looking for any signs of wear? Have it analyzed and see if any red flags show up. If there are some wear signatures, see if the dealer might be convinced. If nothing else, if the trans does fail later you have all the prior reports AND a lab report saying "we saw abnormal wear" prior to the CPO. Just a thought.

And if it is totally clean, it might assuage your fears you are headed to a failure....