My '90 525i (M20B25) consistently hesitates very briefly when accelerating from stopped after it's warmed up for about five minutes or so, and when the outside air temperature is less than about 5 degrees C. It's not enough to be more than vaguely irritating, though it did stall once when I was a bit heavy on the throttle. It doesn't do it when the engine is completely cold, or after about ten minutes of running.

Any thoughts on what might be causing this?