Only other thing I can think of is moisture (water) in the fuel. Not enough to keep it from running but enough to make starting hard. In cold climates condensation in the tank can cause problems. I can't remember what it's called but there is something you can add to the fuel to remove the water.

I'm anxious to hear what the problem is.

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'05 X3 3.0i Alpine White/Gray Leather/Alum. Trim
Sport/Steptronic/Servotronic/Panoramic
Prem. Sound/Staggered 18" Style 114 Y-Spokes