I have not seen a failure on an X5 yet, but check the "Speedometer Impulse Sender" That has very similar symptoms as you descried on E36 and E46. It’s located on the rear diff housing on the 3’s. I don’t have my X here at work today or I’d go check the location for you.

The other option is the dreaded cluster failure that is so common on E36. Again I have not seen it hit the X5 yet.