do you get any other symptoms - e.g. strange noises when you try an accelerate? It probably needs plugging into a diagnostic machine to reveal the issue - any decent independent service garages near you?
you might want to ask in more general BMW forum as it is likely a general issue as opposed to a specific X3 one
http://www.bmwland.co.uk/forums/viewforum.php?f=1
http://www.pistonheads.com/gassing/f...=72&mid=339769