That used to happen to me in my Disco I. What I found was that there is a little bypass channel at the start of the intake, right underneath that idle allen screw that you aren't supposed to touch, that get's clogged after a while. you can see it by taking off the hose section between the MAF and the intake, and looking into the chamber where the throttle plate is. When the throttle plate is fully closed (at idle) that little bypass allows enough air to get in to maintain idle. when it gets' clogged, no air.
get some carb cleaner, stick the straw on it, and shoot it in there, and watch the gunk come out. Also, as Jim mentioned, clean off that stepper motor, because it is surely gunked to, and that little chamber leads to the stepper.