wow that sucks... especially on a 06!! hope everything works out!

my take on this...
 

if it happens only in winter, then the bowden cable (part #5 on the picture) need lubricant. Most likely moisture gets inside, in between the cable and housing. I've seen a post long time ago saying that this is known problem on X5 01-03.

If it happens prety consistently then the actuator is about to go or gone already.

I'll do mine when I got the chance, I just got a tool kit from Bavauto to remove the door panels. I might have to do all 3 doors (the driver side is OK). So hopefully I can tell you guys again what I find.

BTW, my dealer told me they did fix my doors as part of good-will on BMW part (out of waranty but very low mileage), either the dealer lied or or they just reset the fault cause it was OK for 5 months (summer) then problem come back in winter (and only during winter).

cool, thanks