I have a 2005 X5 4.4 that experiences somewhat similar problems. One day it simply decided not to start. It turned over, but the fire just wouldn't catch.

It was towed to the dealer, who took a week to replace an actuator on the starter. Picked it up and made a couple of stops before finally parking it in the driveway. The next evening — the same problem. Simply wouldn't start. I took out the key and tried again. Moved it to neutral. Tried it different times. Towed to the dealer again — where it started with no problem (multiple times).

It's now getting to the point where I'm going to worry each time I take it out. Will it or won't it start.

I too suspect some sort of electrical problem (grounding or some such thing). Would love to hear from anyone else who had a similar problem and how it was resolved.