I noticed this thread...dead thread?...and found it interesting since my own 2K Ford Focus wagon has exhibited a similar problem for the past three months since reaching approx. 78,000 miles--rough idling followed inevitably by stalling and accompanied by the yellow "check engine" light. The latter didn't come on until after the first month or so, but has stayed on ever since. I didn't take the time to fix the problem because it was really more an inconvenience than a debilitating issue--that is, until the missus stalled out in slow traffic on the interstate. So then I got serious with my troubleshooting skills.
To my credit I'd already replace the worn spark plugs immediately after the first signs of poor idling. Didn't change anything but I felt better anyway. So I thought the next step would be cleaning the fuel injectors or having Ford replace the recalled fuel pump mentioned herein. Either of these steps was too time consuming to fit easily into my schedule so I procrastinated. My recent awakening as to the importance of fixing the problem motivated me to do a web search and I found this thread. After reading the comments contained herein I decided to replace the fuel filter--a step that was easily and cheaply accomplished, even on a Sunday. Yes, there was a lot of discolored crap in the old filter (I don't think it had ever been replaced), but no, changing it didn't solve my problem.
That's when I first really paid attention to the engine's noises and remembered one of the first suggested steps in my Ford Escort manual for troubleshooting an idling problem--listen for a vacuum leak. Sure enough, I could hear what seemed like an unduly loud hissing sound which seemed to originate from somewhere behind the intake manifold. After a few minutes of feeling around for a hose of connection that might be the source of the sound, I discovered a deteriorated rubber elbow connected between a hose coming off the right rear side of the rocker cover and an unidentified manifold connected to the rear side of the cylinder head. The rubber had become soft and too pliable and the force of suction had created a hole in the side of the elbow. Although it is a dealer part and one which I will not be able to replace until after Labor Day, a careful application of duct tape has temporarily cured the problem and my Focus now idles perfectly again. Total time of repair (not including unnecessary measures taken): 15 minutes; total cost of necessary parts/mat's: $0.25. Not bad for an amateur!
Anyway, the main reason for my reply to this thread is to inform other Focus owners that this rubber elbow is highly suspect as an 80K maintenance item since I believe it's deterioration is due to poor design and inadequate materials. Moral: if you hear the hiss, heed this heads up!