this same problem happened to my wife's bmw X5. i was running a k&n filter for over 40,000 miles. the truck was stalling quite a bit & the idle would also surge & fluctuate on occasion.

the solution...a new mass air flow sensor. reason...excessive oil build-up in the sensor & screen (mas) due to "over oiling" the freshly cleaned k&n filter.

the mas was replaced and stock air filter put in place again & i have never experienced this problem again.

i would bet this is the "real" problem.

hope this helps.